GGWB is an acronym for girls with above average gluteus maximi in Will Smith's (of Fresh Prince of Bel-Air) Chicktionary...

Good
God
Whatta
Booty

Source: Fresh Prince of Bel-Air - Take My Cousin...Please; Episode 11, Season 4

Episode Summary:
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...