Green’s Dictionary of Slang

1. one who has had their hair straightened with a ‘process’ haircut.

[1842		Flash (NY) 4 Sept. n.p.: Whether that half baked dough baby and the gas head jeweller really think that E.O. [...] seeks to out them in the Flash].
1968		‘Sl. of Watts’ in Current Sl. III:2.
1972		D. Claerbaut Black Jargon in White America 65: gashead n. 1. one who has a processed or straightened hairstyle.
2000		Ebonics Primer at www.dolemite.com 🌐 gashead Definition: someone who has had their hair treated (usually by an amateur), with gasoline in order to eliminate the hair’s natural kinkiness. Example: Look under his hat and you see a gashead.

2. in fig. use, a black person who rejects their racial origins.

1972

D. Claerbaut Black Jargon in White America 65: gasheadn. […] 2. a black person who is a traitor to the pride of his race.