Gunn was born in the Badlands, a section of inner-city Los Angeles where the police would not go, and looked after his sister, Alonna, from a very young age. Although he had a few brushes with the law, he acted as a kind of urban Robin Hood to keep the streets in his neighborhood safe. In his teens, Gunn rose through the ranks to become the leader of a gang of street-fighters who protect their turf from vampires using guerrilla tactics. Possessing the mind of a military strategist and the strength of a back-alley brawler, life in the ghetto hardened Gunn to the extent that his life became less important than the cause, resulting in him trading his soul for a truck in a deal with a demon named Jenoff when he was only seventeen, because he believed he had no future (cf. "Double or Nothing").

Gunn is first introduced in the Season One episode "War Zone", when he observes the vampire Angel roughing up a blackmailer and tries unsuccessfully to dust him.

Eventually, Angel wins Gunn's trust, but unfortunately not in time to save Alonna from being turned into a vampire. Gunn has no choice but to stake his own sister, a decision which continues to haunt him through the series. It is this loss that forces Gunn to question his own motives and become more receptive to Angel's help, realizing he cannot do everything alone.