Unlike most Western animated TV series where characters either have 4 or 5 fingers, Gravity Falls manages to mix characters of having either amount. According to former series writer/ creative director Micheal Rianda, this is because some characters look better having 4 or 5 fingers on them.
The show uses a variety of codes and ciphers for viewers to crack in order to uncover secret messages at the end of each episode. Sometimes the messages are comical, but other times a message will reveal hints and important clues for future episodes.
The symbol in Stan's fez (a form of crescent shape) was changed in the middle of Season 1, due to an issue with another country where the symbol was considered offensive. In the episode Boss Mabel, the goat Gompers ate Stan's fez during the credit scene. In the next episode Bottomless Pit, Stan appears with a different symbol on his hat (a shape resembling Pac-Man). This same symbol was also used in the opening sequence. Oddly enough, the symbol was not changed in the picture with Bill Cipher's wheel displayed during the final split-second frame of the intro.
Bill Cipher, the dream demon, can see through his depictions (a triangle with an eye). His depictions are everywhere in the show, which means he has an appearance in every episode of the show. The crew behind the show put his depictions everywhere so the audience knows that he is watching the Pines family ALL the time.
In the episode "Golf Wars" when Big Henry goes to push the cart throught the gas leak is referencing to the song "Big Bad John" which is about a big quiet guy who saves everyone in a mine that is collapsing risking his life.
A supernatural creature highlighted in the show's mini-series, "Dipper's Guide to The Unexplained", called the Hide-Behind, was described first in a manner similar to the popular urban legend, Slenderman. When the Hide-Behind's physical appearance is revealed, it more closely resembles some sort of cross between Marvel's Groot from Guardians of The Galaxy and the aforementioned Slenderman. On a similar note, a popular Gravity Falls meme was Slenderman making numerous vague appearances throughout episodes, which showed him photoshopped into episode screen caps, despite never making an official appearance.
Alex Hirsch, the show creator, provides the voice talents of the following characters: Grunkle Stan Pines, Soos Chambrot, Bill Cipher, Fiddleford Hedron "Old Man" McGucket, the Gnomes, Quentin Trembley III Esquire, Paper Jam Dipper, a few of the members of the in-show boy band Sev'ral Timez and numerous supporting characters.
According to the shows art director, the point of view its actually Bill. Whenever Bill appears,he has agents watching. That goes for the sayings that bill is watching. You can tell it's bills point of view because in some episodes the screet blackens fro half a second, letting the audience know Bill is blinking.
In the episode 'Love God' there was supposed to be a lesbian couple at the diner scene and the Love God's necklace would've been the LGBT symbol but for obvious reasons, Disney censored these ideas. In the end Mabel's sweater ended up being a rainbow sweater.
The names of multiple character make references to things in real life. Dipper is named after the Big Dipper constellation(and is actually his nickname, due to him also having the Big Dipper in the form of a birthmark on his forehead), Bill Cipher is named after the dollar BILL and the CIPHER wheel, and Pacifica Northwest is named after The Pacific Northwest.
At the end of the opening a picture is displayed. There's a wheel with Bill Cipher in the middle and symbols around him. Each symbols represent a character in the show, though it's not known what is the purpose of it yet and some symbols remain a mystery. The picture also shows the Konami code and a cryptogram that says "Stan is not What He Seems", this is a reference to Twin Peak's "The Owls are not What They Seem" and also a foreshadowing to the episode "Not What He Seems".
In the closing credits of each episode, there is always a cryptogram that, when deciphered, usually references to something that happened in the episode. There are 4 different ciphers that are used: Caesar, Atbash, Vigenére and A1Z26.
The Big Foot can be seen in the opening for a brief moment. This is brought up by Bill Cipher in the episode 'Dreamscaperers' when he displays a sequence of images showing that he knows lots of things though it probably is just an Easter Egg.
Creator Alex Hirsch actually inserts himself within the show several times throughout the series. One example present in all episodes, however, is a picture on the top of the screen in the closing moments of the theme song when all of the pictures are being dropped into a pile. Another, exclusive to one episode is in Society of the Blind Eye, Season 2 Episode 7, where a photo of Alex in the art style of Gravity Falls is pinned to Dipper's board of suspects for being the author of the journals. His photo is circled in red with "Who?" written over it.
Some time after the airing of Episode 19 of Season 2(Weirdmageddon 2: Escape from Reality), Alex Hirsch himself confirmed the ending of Gravity Falls in the form of the hour-long Season 2 finale, which would conclude the Weirdmageddon story arc and the series as a whole. An official article concerning this news detailed that Alex intended for Gravity Falls to end at the peak of it's creativity rather than spanning for multiple seasons or episodes until it faded into obscurity like numerous other children's cartoon shows.
The Time Baby is "the last son of an extinct race of time giants" (according to Bill Cipher). According to the secret document Dipper and Mabel were reading in the episode 'Irrational Treasure' he has been trapped in a glacier in Antarctica for a really long time. This document foreshadows his first appearance in the next episode 'The Time Travelers Pig'.
In Season 1 episode 9 titled The Time Traveler's Pig. Two time police return for Blandin to account for all the disruption caused in time travel. They are dressed like Universal Soldiers. One is named LOLPH and the other is named DUNDGREN which is a nod to "Dolph Lundgren."
The TV in the Mystery Shack is produced by the electronics manufacturer and corporation Worrel which is named after Ian Worrel, the art director of the show, who won an Emmy for his work in the 18th episode of the first season, Dreamscaperers.
"Gravity Falls" has two ties to Scooby-Doo. The character of Wendy is voiced by Linda Cardellini, who played Velma in the two live-action theatrical Scooby-Doo movies, and Grey Griffin (formerly Grey DeLisle), who voices Daphne in many of the Scooby-Doo animated movies, provides additional voices on a regular basis for "Gravity Falls".
Mabel Pines actually exists and she's called Ariel Hirsch. She's the twin sister of Alex Hirsch. Mabel is based on Ariel, Dipper on Alex, Grunkle Stan on Alex's grandpa Stan and Soos on a friend of Alex from college called Jesus Chambrot. Also Soos' address is 32 Chambrot Drive, Gravity Falls, which is a reference to his friend surname.
There is a icelandic pop group called BABBA, who sing a song named Disco Girl. There is a band in real life named ABBA who sing Dancing Queen. Disco Girl is a parody of Dancing Queen and BABBA is a parody of ABBA.
The trivia items below may give away important plot points.
Gravity Falls is assumed to reside in the same canonical universe as the Adult Swim TV show Rick and Morty. During the ending bumper for one episode of Gravity Falls, Stan's coffee mug, notepad and pen fly through a portal he is monitoring. In an episode of Rick and Morty, the same three items are seen falling through a portal in the background.
Blendin Blandin's first official appearance is in "The Time Traveler's Pig". The next episode he appears in is "Blendin's Game". However, if you look closely, you can see Blendin standing behind a bush in the forest in "Tourist Trapped", amongst a crowd of people for a brief moment in "The Mystery of The Gobblewonker" and at the end of "Gideon Rises", all before running off somewhere. All these appearances are revealed to be more than Easter eggs, and is actually him travelling back to numerous points in the past to amend time anomalies caused by Dipper and Mabel throughout the course of "The Time Traveler's Pig".
The evil triangle demon, Bill Cipher has a section in both journal 2 and 3, but journal 2 contains instructions on how to summon Bill journal 3 contains instructions on how to stop him if he is summoned. Additionally journal 2 states that his name is unknown while journal 3 gives the name Bill Cipher.
Before the start of Season 2, an image was leaked online, depicting a slightly younger version of the character Old Man McGuckett, sitting at a desk of sorts writing in a journal. He also appeared to have six fingers on one hand, suggesting that he wrote the three journals seen throughout the show, which all had a golden hand with six fingers on the cover. However, sometime after the airing of Episode 7 of Season 2, where it is revealed that Old Man McGuckett didn't write the journals, it was revealed by Alex Hirsch that the leaked image of McGuckett was actually a hoax, despite it being an official image by the animators and artists.
A page from one of the journals in Gravity Falls shows a wheel of sorts encompassing a drawing of the character Bill Cipher. To the right of this wheel, a cryptogram can be found which translates into "Stan is not what he seems." This cyrptogram reappears after the credits of Episode 10 of Season 2, repeated thrice in the form of a combined cipher, and again as the title of Episode 11 of Season 2, "Not what he seems", 'he' again referring to Grunkle Stan.
A popular theory about the series was that Grunkle Stan had a twin brother who disappeared at one point before the events of the show and had some connection to the three Journals that appear throughout. This theory was almost rendered null and void when a leaked photo from Season 2 showed a younger Old Man McGuckett writing in a journal, with six fingers on his right hand. Up until "Society of the Blind Eye", the idea that McGuckett wrote the three Journals was basically canon, until it was revealed that he only worked with the true author of the Journals. In the mid-season finale of Season 2, "Not What He Seems", it is revealed during the end that Grunkle Stan did in fact have a twin brother who was lost in the portal underneath the Mystery Shack, and wrote the Journals.
All throughout Season 2, at various points in episodes, random words can be found etched into the background. These words are actually keys for solving Vigenère ciphers provided in the credits. Along with these ciphers, combinations of Atbash, Caesar and numbered Ciphers are seen after the credits in sections of a 'Cryptic Page'. These sections supposedly fit together to create the Cryptic Page, with what is believed to be Grunkle Stan's face in the centre of it.
The number 618 appears multiple times during the show, which is a reference to Alex Hirsch's birthday, June 18th. Examples of this include the number on the cash register in the Mystery Shack, the number painted onto the back of Gideon's truck in Weirdmageddon Part 1 and the experiment number for Uncle Ford's Quantum Destabilizer from the same episode.
The fact that Dipper can be seen in the Gravity Falls Gossiper fighting a supernatural creature at the beginning of Northwest Mansion Mystery means that the Society of the Blind Eye has no longer any control over the memories of people in Gravity Falls.
In the episode 'Dreamscaperers', Bill Cipher shows a sequence of images to prove that he knows "Lots of Things". One of these pictures show a fallen hand of a robot which is foreshadowing of the next episode 'Gideon Rises' where Gideon lost the fight against Dipper and his plans fail. It seems that Bill Cipher knew that Gideon was going to lose. The sequence also show Grunkle Stan entering his secret lab also foreshadowing the next episode where it's revealed what is behind the vending machine, the Gravity Falls Prison foreshadowing Gideon arrest among other images.
During a scene in Weirdmageddon Part 1(Season 2, Episode 18), Dipper, Wendy, Gideon and Ghost Eyes go through a series of media changes as effects of passing through Weirdness Bubbles. The last change in media we see for Dipper and Wendy is a quick live action shot of two seemingly random actors. These people are in fact Jason Ritter and Linda Cardellini, dressed as their respective characters in the show.
Many episodes have alluded to an oncoming apocalypse. Bill Cipher references the onset of "something big" in Dreamscaperers and Sock Opera, he also tells Gideon that he has been working on something and describes "a darkness approaching. A day will come when everything you (The Pines family) care about will change." In the episode "Into The Bunker" it is possible to see that the Author predicted a catastrophic event and prepared enough supplies to last him several decades.
The inspiration for Dipper's birthmark came from Alex Hirch's childhood. When one of his friends had a huge acne problem he would find constellation in his forehead. One day he found that his acne formed a perfect Big Dipper.
CONTINUITY: In the episode 'The Hand that Rocks the Mabel', after the deal between Grunkle Stan and Bud Gleeful (Gideon's father) go wrong, Grunkle Stan steal a painting from Bud and ran away. He puts the painting in a room in the Mystery Shack. In the episode 'Gideon Rises' after Gideon takes over the Mystery Shack, Bud is seen taking the painting back for him.
During Weirdmageddon, the usual intro for the show is replaced with Bill Cipher's own twisted version, due to the start of his apocalypse in the previous episode. The whisper at the end of the intro is retained but this time sounds like "I'm watching you nerds!" when played backwards, and of course is in Bill's voice. This quote is most likely a shoutout to the Gravity Falls fanbase from Alex Hirsch to them.
The chase scene near the end of Weirdmageddon Part 1 where Dipper and Wendy are pursued across a desolate landscape is a reference to Mad Max: Fury Road, as well as one of Gideon's discount Road Warriors resembling Max.
Before the release of the episode, the San Diego Comic Con trailer for Season 2 showed numerous snippets from "Soos and The Real Girl", namely shots of animatronics coming to life an attacking the main cast. Since then and up to the release of the episode, many have believed it to be a direct reference to the popular horror game franchise, Five Nights at Freddy's. It has however been confirmed that this is not the case, as the idea for the episode as well as numerous scenes had been animated and finalized before the release of the game.
In his previous appearances, the character Bill Cipher has been shown to be able to separate his top hat from his body, as expected. Until it was revealed in Weirdmageddon, when Uncle Ford fired his Quantum Destabilizer at Bill only to miss at the last second, that the hat is actually a part of the triangular demon, complete with fleshy insides and even a skeleton of sorts.
For almost all promotional previews for future episodes of Gravity Falls, a split-second shot of the Cipher Wheel from the Journals written by Uncle Ford can be seen right after the Disney XD or Disney Channel title card. After Episode 17 of Season 2, Dipper and Mabel vs The Future, promotional previews for Episode 18, Weirdmageddon Part 1, show this Cipher Wheel slightly differently, as the drawing of Bill is replaced with a ripped hole in the shape of him adorning the page instead. This is a reference to how Bill Cipher had finally escaped from the Nightmare Realm and destroyed the rift between his dimension and Earth's, allowing him to truly take form.
In the Season 2 promotional poster, Old Man McGucket's hat and Grunkle Stan's hat can be seen lying on the ground, possibly referring to either one of them dying in the series, supposedly in the season finale.