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.
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 first name of the main character Dipper Pines is actually a nickname he recieved due to a birthmark on his forehead that looks like The Big Dipper. His actual first name has yet to be revealed. It was later after the shows end that Dipper's first name was revealed to be Mason.
Alex Hirsch, the show creator, provides the voice talents of the following characters: Grunkle Stan Pines, Soos Ramirez, 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.
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 a 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.
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.
At the end of the intro song, there is a whisper that sounds like "I'm still here." Reversing the audio reveals that it says "three letters back." later it says "switch a with z" after that it says "26 letters" in season 2 it says "key vigenère" all of these are hints to the cryptogram at the end of the episode in the episode "not what he seems" the whisper is changed to "not what he seems"
At the end of the opening intro, a picture is displayed of a wheel with Bill Cipher in the middle and symbols around him. Each symbol represents a character in the show, though the meanings behind the symbols are not revealed until the series finale. The picture also shows the Konami code and a cryptogram that says "Stan is not What He Seems." This is a reference to "The Owls are not What They Seem" from Twin Peaks and is a foreshadowing to the episode "Not What He Seems."
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 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.
Creator Alex Hirsch appears in 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. He also appears within the show several times throughout the series.
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 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.
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.
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 19th episode of the first season, Dreamscaperers.
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.
Out of the whole staff of the Mystery Shack only Soos has no criminal record. Dipper and Mabel have been arrested for counterfeiting, Wendy has stolen a car, and Stan has committed too many crimes to list.
"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".
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".
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
After the theme song Bill Cipher is in the center on a ring of symbols each representing characters from the series. It appears again in the finale and the characters representing the symbols are revealed
Even though Gravity Falls ended at the beginning of 2016, Alex Hirsh, who created the show, is still releasing merch (Journal 3, Dipper and Mabel curse of the time pirates treasure, Mabel's cursed coloring book, Journal 3 special edition) which could be a hint that Alex Hirsh might continue the show.
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.
In a season 2 promotional poster the kids are show in front of the portal. The shadow of the portal happens to be Bill Cipher. Indicateing that not only will he be a part of season 2 but he also has a connection to the portal.