Gay Guy is a deaf guy that always is over at Marcus' house. He's deaf, small in stature, his real name is Mike, and he resembles Pete Postlethwaite (Kobayashi from the movie The Usual Suspects). When they first saw him, Marcus made up a story about how he tried to rape some kid so he was going to jail, and that he's gay and dying from a rare throat cancer. Marcus finally revealed that he was just joking, however Mike's sexually remains ambiguous. The name gay guy comes from a time when Marcus was trying to get the guy's attention, so he said "Hey Mike... Mike! GAY GUY." Of course, he couldn't hear Marcus because he's deaf.
The reason he's always over at Marcus' house is that he "fixes stuff" in return for food and money. Adam always wondered if he ever ran out of stuff to fix.