Gilbert "Gil" Grissom was born on August 17, 1956 in Santa Monica, California and grew up in Marina Del Rey, California. His father, a botanist, died when Gil was nine years old. His mother, who was deaf, ran an art gallery in Venice.
Grissom appears to have both moral and intellectual stakes in working as a criminalist. While he rarely, if ever, becomes emotionally involved in cases, he often makes pithy indictments or offers pointed quotations to illustrate his reaction. He was raised Roman Catholic, but he now seems to be a lapsed Catholic. He describes this as "practic[ing] a kind of secular Catholicism that involves ritualizing certain aspects of everyday life and imbuing them with a spiritual intensity they might not otherwise possess". This appears to neatly sum up his personal faith in scientific rigor as well as humanity, despite the crime scenes he attends every night. In an eighth season crossover arc, he confesses to FBI Agent Jack Malone that he thinks about "hanging them up" every day, but that he has "his pig" - in other words, the work still holds unanswered questions and fresh insights for him. Shortly thereafter, he agrees with colleague David Hodges' description of the work as "a puzzle" that he's not yet ready to let go.
Grissom describes himself as "a ghost" in school, focussing on his studies and on his scientific interests. He was known to pick up and dissect animals killed along the highway, and was eventually called upon to assist local law enforcement with necroscopies and other forensic tasks. (It is hinted in the second season that Grissom has Asperger's Syndrome, or Asperger's-like tendencies, and these are key examples: social withdrawal and intellectual obsession leading to mastery of a subject.)
Grissom has a degree from UCLA in Biology, and completed his PhD at UC Berkeley. He enjoys teaching, perhaps in part because it provides him with a structured social setting in which he can find success through sharing his interests. He later take a professorial role with his crew, mentoring them through their career developments and imparting wisdoms gleaned from years of experience. As the crew draws near his level of expertise, he finds himself excited to be offered a teaching sabbatical, explaining: "I'm a teacher without any students."
At only 22, Grissom became the youngest country coroner in California history. Winning a surprise windfall in a college poker game, he set up his first Body Farm - a natural setting where donated human bodies could be observed and recorded at all stages of the process of decomposition, local predation, and of course, bugs - entomological interference.
At 29, Grissom switched careers and became a CSI, rising quickly through the ranks to become a shift supervisor. He eventually transferred from the San Francisco PD to the Las Vegas PD, and took over the graveyard shift. Under his guidance, the LVPD Crime Lab rose to a second place crime-solving position in the country, second only to the well-funded FBI national headquarters at Quantico.
Grissom, while attached mainly to his work, does make friendly connections with his staff. And at least one friend is willing to relocate entirely to work with him: CSI Sara Sidle appears in the second episode of the series, called in from San Francisco by Grissom to run an internal investigation. His night shift crew treat him as an avuncular figure, eccentric but wise, hilariously dry, and solid as a rock. His own supervisor, Conrad Ecklie, considers him the proverbial thorn and a possible contender, and takes every opportunity to put him in his place.
Grissom's inherited his mother's otosclerosis, a fusion of the small bones in the ear, which nearly led to the loss of his hearing. This leads him even further into himself, withdrawing emotionally from his crew, and especially from Sara, for whom he has feelings he can neither deny nor act upon. With successful surgery, he regains his full hearing, but the effects of his behaviour upon Sara take much longer to heal.
Grissom and Sara, while ferociously intelligent and adept observers, both find personal relationships awkward and difficult. Grissom especially takes a great pleasure and pride in his powerful mind, and doesn't often see the point in changing his emotional habits to accommodate the people around him. Sara, having overcome years of trauma to succeed in her chosen path, surrounds herself with emotional brick walls. It takes years for them to realize that neither of them is asking the other to change - only to be honest and willing to try. Six years after she arrives from San Francisco, Grissom and Sara are finally shown to be a couple, though they keep their relationship hidden from their colleagues for another two years. It is not until the end of the seventh season that Grissom reveals to his colleagues, during Sara's kidnapping, that she is the only one he has ever loved.
Upon Sara's return, she and Grissom must deal with the administrative effects of an interoffice relationship. Sara volunteers to switch to the swing shift, which brings "more daylight" into her life, but which leaves them with little time to spend together. Grissom, in his usual sideways manner, suggests that perhaps they should marry, and Sara agrees delightedly. This seems to boost her spirits, but even Grissom can't entirely stop her headlong spiral into a PTSD-sparked depression due to her violent childhood and more recent ordeal. She leaves him to return to California to confront her ghosts, while Grissom, giving her the space she asks of him, throws himself entirely back into work.
Season Eight shows Grissom in emotional retreat, occasionally speaking with Sara by phone or video, but otherwise channelling all of his energy into his work. His team is fraying around the edges, with Sara gone, Ecklie hovering, and Warrick's problems affecting his work, and he feels responsible. Warrick's untimely death and Sara's brief return for the funeral act as an emotional catalyst upon him, and within a few months, he makes the decision to retire as CSI.
Having made the grand gesture of reuniting with Sara in the jungles of Costa Rica, they were quietly married between the ninth and tenth season. Season Ten opens with the news that they have been living in Paris, where Grissom is teaching at the Sorbonne while they wait for their recent research proposal to be funded. Though Sara, apparently bored with waiting for something to do, returns to her old job in Vegas, she returns to Paris to visit him from time to time, and they appear to be one another's strength and stability, as ever.