Gil was raised by his mother, who was deaf, after his father died when he was nine. Never good at making friends, he was a social recluse while growing up. He graduated early and became the youngest Los Angeles Medical Examiner at 19. Decades later he moved to Las Vegas, taking the Las Vegas Crime Lab from near the bottom of crime lab rankings to the #2 Lab in the country, second only to the FBI lab at Quantico.
Grissom is a renowned forensic entomologist and for fun likes to race Madagascar Hissing Cockroaches.
He took over the Night Shift as Supervisor after Brass was demoted to Detective after the death of Holly Gribbs. He has tried to instill in his CSIs the ability to find a balance between science and emotion, at times losing that balance himself.
Gil doesn’t talk about his obsession with Nico during the years in Vegas. It was this obsession that had him researching things into Nico’s past, discovering the world of Vampires. He was pulled into the world when his team was attacked by Rogues who were looking to harm Nick Stokes and Warrick Brown, the leaders of Sylum Clan. After discovering about Vampires he was pulled into a Clan War. He soon saw the men he trained in a different light.
His life once again changed when he had a confrontation with Harvey Spector.