I learnt the meaning of Gibby from American Hardcore - it's made it into the Urban Dictionary now:
... a large amount of crack cocaine. According to the book American Hardcore a Tribal History by Steven Blush, Gibby Haynes was a very prominent member of certain crack circles in parts of texas. In these said parts you can still purchace a "gibby" which is a large bag of crack. no specfic weight given just large.