Green’s Dictionary of Slang

(US black) to straighten one’s hair.

1948		Roy Porter [instrumental title] Gassin’ The Wig.
1953		L. Durst Jives of Dr. Hepcat (1989) 7: Well I guess I’ll ooze on down to the crib and get on the beam for the hop tonight. I’ve got to take a rub down in water and you must believe l’ve got to gas my moss.
1969		Frank Zappa ‘Willie the Pimp’ 🎵 I’m a little pimp with my hair gassed back.
1980		E. Folb Runnin’ Down Some Lines 241: hair, conk/fry/gas/press (one’s) Straighten one’s hair.