Gregory is a product of shady genetic experiments by Fazbear Entertainment, grown from the DNA of the late Franky Fazbear (God rest her soul.) With Franky dead by the time Gregory was born he was instead given over to Raelynn Lawrance, one of the therapists under FazEnt payroll. Raelynn would act as Gregory's primary guardian until her disappearance.

Due to abnormalities and alterations in Gregory's genes, Gregory has extra teeth and is able to contract or dilate his pupils on command. This is equivilent to messing with the brightness and gamma levels in-game.

Raelynn's therapy office was a part of the house, and Gregory saw Vanessa often when she came in for her sessions. Vanessa was the last client Raelynn had before going missing which is the source of Gregory's fear of her. He ran away from home shortly after, convinced that Vanessa would come back to finish the job.

Gregory keeps all of his possessions in a Pizzaplex duffel bag, including a folded up cardboard box big enough to hide in.

In actuality, GGY killed Gregory's mother and Gregory simply doesn't remember. Nobody involved knows enough to tell him the truth.