Green’s Dictionary of Slang

(US black) used of hair that has been chemically straightened.

1965		B. Jackson Get Your Ass in the Water (1974) 162: You could tell he was a pimping motherfucker by the way his hair was gassed.
1967		A. Baraka Tales (1969) 76: Gator would strut up and down the stage [...] shaking his long gassed hair.
1980		E. Folb Runnin’ Down Some Lines 141: It was still not uncommon to hear teenagers talking about getting their hair [...] pressed, or gassed.
1987		J. Ellroy Black Dahlia (1988) 278: I smelled reefer smoke on the sidewalk outside the gassed hair joint on 68th and Beach.