GGWB is an acronym for girls with above average gluteus maximi in Will Smith's (of Fresh Prince of Bel-Air) Chicktionary...
Source: Fresh Prince of Bel-Air - Take My Cousin...Please; Episode 11, Season 4
Hoping to raise his grades, Will lends his divorced and depressed professor his "chicktionary" of hot dates. Each name has a letter grade next to them and the professor sees a few GGWBs and asks Will what it means...
Will's Professor - What's this GGWB?
Will Smith - Oh, thats "Good God, What a Booty"