Green’s Dictionary of Slang

(US black) a term of address to a male friend.

[1879		in W.A. Graham Custer Myth (1953) 333: Benteen then faced me and said [...] ‘“God” (my nickname among my fellows), I could tell things that would make your hair stand on end’].
1997–2000		Da Bomb 🌐 13: God: (noun) Someone for whom one holds high regards and deeply respects as a friend.
2000		P. Beatty Tuff 167: ‘What, she think I want to know that shit?’ ‘Why you think, God.’.