Gordon Walker was 18 years old when a vampire broke into his house and attacked his sister. Gordon grabbed his father’s shotgun, attempted to free his sister, and then finally shot at the vampire. The vampire threw him across the room, knocking him unconscious, and abducted his sister. When Gordon regained consciousness, the vampire and his sister were gone.
Gordon ran away from home learning to fight, hunt, and kill vampires. He eventually found the vampire that had taken his sister and killed it. He also killed his sister, as she was then a vampire. This marked the beginning of his hatred for the undead.
Gordon was introduced in the episode "Bloodlust". Dean and Sam Winchester meet up with Gordon Walker while hunting some particularly vicious vampires. Sam calls Ellen and asks her what she knows about Gordon. Ellen tells him that Gordon Walker is a very good hunter, but to stay away from him, as he is dangerous to everyone and everything around him. Gordon Walker tries to attack a nest of vampires even though Sam proves they do not attack humans, drinking blood from cattle instead. Dean ends up beating Gordon in a fight leaving him tied to a chair.
In the episode "Hunted" it is revealed that Gordon Walker was performing an exorcism on a girl when the demonic entity living inside her mentioned something about a war. Gordon managed to get more information out of it, including the fact that Gordon personally knew one of the people who were to be involved in the war. The girl being exorcised was "questioned" by Gordon Walker and died because of it.
He then searched for these people, killing one about a month before the episode begins. He tracks Sam Winchester and tries to shoot him, but Dean Winchester intervenes. After a scuffle Gordon knocks Dean out, ties Dean up, and tells him his plans to catch Sam in his booby trap: grenades set with trip wires. Sam manages to evade the trap and knocks Gordon out. As Sam and Dean leave the damaged house, Gordon is back up and chasing them, firing two pistols in their direction. The police arrive to subdue Gordon, and then find the cache of weapons in his car. Sam Winchester notes that Gordon could possibly escape from jail. He is well-known for being ruthless, pragmatic and incredibly resourceful.
Gordon appears again during the episode "Bad Day at Black Rock" in jail and talking to a fellow hunter whom he convinces to go after Sam Winchester. He tells him that he is not even sure Sam Winchester is even human. Gordon Walker is aware of the brothers having opened the Devil's Gate in the second season finale. At the end of the episode he tells the hunter he was talking with that they will have to break him out of prison.
In the episode Fresh Blood, Gordon pursues the Winchester brothers again. During the hunt, he is turned by a particularly old vampire named Dixon who has a grudge against hunters. He kills two other vampires turned by Dixon, then sets up a trap for the Winchester brothers, still convinced it's his duty as a hunter to rid the world of Sam Winchester. A fight ensues in a warehouse where Gordon holds a girl he has kidnapped and turned into a vampire to use as bait. Sam Winchester decapitates Gordon Walker with a garotte improvised out of barbed wire during the struggle, thus ending the threat he poses.