Green’s Dictionary of Slang

(drugs) a barbiturate, a depressant.

1966		J. Mills Panic in Needle Park (1971) 32: He was high on barbiturates, goofballs, GB’s. Barbiturates are sleeping pills, but taken excessively they may produce not sleepiness but a frighteningly unpredictable aggressiveness, such as Bob was displaying now. He was, in the junkies’ word, ‘goofing’.
1969		R.R. Lingeman Drugs from A to Z (1970).
1971		E.E. Landy Underground Dict. (1972).
2001		ONDCP Street Terms 9: G.B. — Depressants.