acronym for "grow a brain". usually used in reference to someone lacking common sense; often used as a stand-alone word as a comeback or a response to a comment or thought made.
Carlos: "Man, Rosie's ass is so fat. It's a shame she is so ugly."
Francisco: "I disagree. Although her face is like that of a retarded troll or hispanic Dr. Suess character, with an ass that fat and tight, she's still a GAB."
Acronym for "gay after beer". Commonly seen among college frat boys who think and act like they're totally straight, yet engage in various homosexual acts once intoxicated.
Guy A: "Dude, that new pledge is so hot...I'd do anything to suck his dick. I wonder if he has a girlfriend"
Guy B: "Oh please...he's a total Gab...just give him a few drinks and he'll be on his knees in no time."