Green’s Dictionary of Slang

gee n.6

[? initial letter g of various opium-related words, e.g. gong n.2 (2); gow n.1 (2); gum n.3 or of SE guy, which is synon. with boy n.2 (5a), i.e. heroin; ? even more far-fetched is poss. pun on SE gee-up!, used of horses, thus linked to horse n. (7), heroin; Maurer, Language of the Und. Narcotic Addict Pt 2 (1938), suggests Hindi ghee, refined butter; also note Chinese yí-yín/gee yen n., seconds, i.e. opium residue]
(US drugs)

1. opium.

[US]D. Maurer ‘Lang. of the Und. Narcotic Addict’ Pt 2 in Lang. Und. (1981) 102/2: gee. [...] Smoking opium, especially refined or reworked opium.
[US]Monteleone Criminal Sl. (rev. edn).
[US]J.E. Schmidt Narcotics Lingo and Lore 66: Gee – A narcotic drug, esp. an opiate.
[US]ONDCP Street Terms 10: Gee — Opium.

2. narcotics other than opium; also attrib.

[US]B. Dai Opium Addiction in Chicago 199: Gee. Drugs in general.
[US]R.R. Lingeman Drugs from A to Z (1970) 101: gee head [...] A paregoric addict.
[US]E.E. Landy Underground Dict. (1972) 87: gee head n. Paregoric addict a.k.a. gheid.

In compounds

gee-fat (n.)

see separate entry.

gee yen (n.)

see separate entry.