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."