Green’s Dictionary of Slang

(drugs) a small dose of drugs used to hold off withdrawal symptoms until a full dose can be taken.

1969		D.E. Smith It’s So Good 109: The small amount of heroin he is given (known as a G shot).
1971		J.T. Dunigan Drug Abuse: A Resource Guide.
1997		L. Pettiway Workin’ It 186: A G shot, that’s less than a tenth. Just to get them, you know, just to get the edgies off.
2001		ONDCP Street Terms 9: G-shot — Small dose of drugs used to hold off withdrawal symptoms until full dose can be taken.