Green’s Dictionary of Slang

(drugs) opium, morphine.

1930		Burlington Dly Times (NC) 19 Dec. 1/8: ‘There is enough morphine there to kill several normal healthy men’ [...] G.O.M. (God’s Own Medicine) is sometimes the devil’s broth.
1938		D. Maurer ‘Lang. of the Und. Narcotic Addict’ Pt 2 in Lang. Und. (1981) 103/1: g.o.m. Morphine. Literally, ‘God’s own medicine,’ a phrase borrowed from legitimate medicine.
1953		Anslinger & Tompkins Traffic In Narcotics 308: G.O.M. God’s Own Medicine: morphine.
1959		J.E. Schmidt Narcotics Lingo and Lore 70: G.O.M. – Good Old Morphine.