Charles Gunn had been fighting vampired on the streets for years. He and his "Family", a group of teenagers striving for purpose, had it down to a science. Hunting and killing Vampires to make the streets safe.
But the life wasn't easy on Gunn. he lost his little sister to the vampires, he traded his soul for a tricked out truck that he used to slay vampires and even lived in for a portion of his life, and he didn't have a real home.
Then he met Angel. Angel and the others from Angel Investigations helped Gunn regain his soul, and saved him and his Family on a couple of occasions.
Gunn eventually joined up with the vampire with a soul, and stayed with them through thick and thin for years. Eventual he fell in love with Winifred "Fred" Burchil, and the two dated until tensions in the ranks of Angel Investigations tore them apart.
When Angel and Co. were given the LA branch of Wolfram and Hart, Attorneys at Law, Gunn cut a deal to inhance his knowledge of the law and various other aspects that related to his job at Wolfram & Hart. however, this deal later lead to the death of Fred, her soul distroyed and her body taken over by an ancient demi goddess named Illyria.
Around that same time, things went downhill for Angel's gang, and in an attempt to shut down Wolfram& Hart once and for all, the crew killed every member of a secret organization called Circle of the Black Thorn, during which Gunn was mortally wounded. In retaliation, the senior partners of Wolfram & Hart sent LA to hell.
In the battle that followed, Gunn was turned into a vampire.
Now, Gunn is the very thing that he's fought his whole life. Confused and upset, Gunn blames Angel. And he plans to make him pay.
After time was reverted by Wolfram & Hart, Gunn became a human again and lost the vampire abilities.