Green’s Dictionary of Slang

(US drugs) an opium addict or habitual user of marijuana, heroin etc.

1936		D. Maurer ‘Argot of the Und. Narcotic Addict’ Pt 1 in AS XI:2 121/2: gowster. A narcotic addict, especially one who ‘uses heroin, morphine, or cocaine’.
1949		Monteleone Criminal Sl. (rev. edn).
1959		J.E. Schmidt Narcotics Lingo and Lore.
1969		‘Iceberg Slim’ Pimp 124: This pretty ‘gowster’ is sure pimping his ass off.