Gunn is a street-wise vampire hunter. Left to take care of himself and his sister at a very young age, Gunn knows of the dangers that roam in dark alleys and fights vampires with his crew.
When he first meets Angel, he doesn't really trust him - hating vampires - but after the dath of his sister, they grow closer and Angel often calls Gunn for extra help. Soon, Gunn is a full-time member of Angel Investigations which leads to tense relationships with his old gang, who resent his new associates and believe he has now forgotten them.
Working with Angel, Gun realises that world is not just black and white, that demons are not all evil and that there is a 'bigger fight'. He also finds new friends in Cordy and Wesley and later falls for Fred after the gang rescues her for an alternate dimension. While the world around them falls apart, they are living on cloud number nine. But as always in the Jossverse, things go sour when Fred finds out that her college Professor was responsible for sending her to Pylea. Consumed by her revenge she plans to kill him, but Gun steps in, refusing to let her become a murderer and kill the man himself. After this they grow more and more apart and eventually break up.
His friendship with Wesley also suffers greatly and the one time partners in arms end up almost being enemies. Wesley's betrayal and his growing feelings for Fred don't go down well with Gunn. But with time, their friendship is mended and by season five they are close once more.
When the gang accepts to take over the L.A. branch of Wolfram & Hart, Gunn feels less out and useless, since he doesn't have any 'special' abilities like the others. When W&H offers him a mental upgrade that would give him an extensive knowledge of the law and W&H inner working, Gunn accepts. Trading his axe and street-wear for a briefcase and a designer suit. He also has access to the White Room and becomes a privileged conduit between Team Angel and the Senior Partners.
However, Gunn soon finds out that his brain implant was only temporary and agrees to manage the legal work to release an ancient artefact from customs in exchange for a permanent upgrade, knowing that one day his actions would come back to haunt him.
And true enough, they do, when the artefact turns out to be the sarcophagus containing the essence of an Old One, Illyria. When it arrives in Fred's lab, she opens it and is infected. The demon burns her up and takes over of her body. Considering him responsible for the death of the woman he loved, Welsey stabs Gunn. While the rest of the gang aren't so extreme, they also consider him partially responsible, as does Gunn himself, who willingly offers to take Lindsey's place in Hell.
He is later rescued by Illyria, who brings him back to W&H, knowing that this means Team Angel is in her debt. Gunn returns to work, although more like his old self, despite his legal upgrade. When the big battle comes, he gladly abandons forms and paperwork to hit the streets and slay some demons.