Adjective. Used to describe something that is so terrible that it transcends being negative, and becomes good again. Usually used in reference to movies that are poorly made that tried to be serious and ended up as humorous.
Hellraiser 4 is a "G'bad" movie.
Snakes on a Plane is NOT a G'bad movie, however, as it was always meant to be a joke.
R. Kelly's "song" Real Talk is a g'bad song.