Gabriel is Troy's brother. He's the only sibling Troy is still in touch with, though they grew up in a large family. Gabe was wounded in World War II and now has a metal plate in his head. The disability money he receives as result of his injury allowed Troy to buy the house that the Maxsons now live in, a source of shame for Troy.
Just before the play begins, Gabriel has moved out to live with a lady named Ms. Pearl. Gabriel is afraid that Troy is mad at him for moving out because now Troy no longer gets the disability check. Troy denies this. However, later in the play Troy has Gabe committed to a mental hospital and again starts receiving half of Gabriel's check. Troy says it wasn't about the money, but we think this seems pretty suspicious.
Because of his head injury, Gabriel thinks he is his own Biblical namesake – the archangel Gabriel himself. Gabe spends a lot of time chasing hell hounds and reminiscing about all the lovely biscuits he's shared with St. Peter. For more on this, and Gabriel in general, check out "What's Up with the Ending?"