Green’s Dictionary of Slang

1. (US tramp) a gallon of liquor.

1929

V.W. Saul ‘Vocab. of Bums’ in AS IV:5 340: Gee—One gallon of liquor.

2. (drugs) one grain, usu. of morphine.

1953		‘William Lee’ Junkie (1966) 156: G . . . One grain. Morphine is the standard for junk measurement. One-half grain of morphine is one ‘fix.’ A capsule of heroin should contain at least the equivalent of a grain of morphine. Heroin is seven times as strong as morphine.
1969		in Smith & Gay Heroin in Perspective (1972) 109: Taste refers to the small amount of heroin he is given (known as a G shot).

3. (drugs) one gram, orig. of heroin or cocaine, and latterly also of cannabis.

1995		Mack 10 ‘Based on a True Story’ 🎵 From a Key to a G, Its all about money.
1997		N. Barlay Curvy Lovebox 25: Course the pukin’s fuck all to do with the two gees o’whizz he does for breakfast.
2000		N. Barlay Crumple Zone 107: Big dick, three gees of kuf an’ air-con in ma car.
2012		(con. 1980s) I. Welsh Skagboys 71: There’s nothing ah can dae, wasted an wi a G ay gear on us.
2015		I. Welsh Decent Ride 31: It’ll cost ye nowt but yir minibar tab n mibbe a couple ay Gs.

4. a girlfriend.

2002

Cypress Hill Sl. Gloss. 🌐 g: girl.

5. grime music.

2005

DJ Cameo on 1Xtra 1 July [BBC radio] Showcasing all the different aspects of UK G.

6. (drugs) gamma hydroxybutyrate (GHB).

2001		ONDCP Street Terms 9: G — [...] Gamma hydroxybutyrate (GHB).
2017		G. Gilmore Headland [ebook] ‘Whatever you need, mate. Charlie, crack, pills, smack, G, P, ice or black’.