Green’s Dictionary of Slang

1. anything that is seen as a male preserve, a ‘guy thing’.

2001

C.B. Cummings Clancy Cummings.net 🌐 Nothing but a ‘G’ thing: Next to marriage, one of the biggest steps in my life appears to be joining the Masonic Lodge. [...] Each of the Brothers I’ve spoken to are extremely happy and enthusiastic.

2. anything that concerns a street thug, a ‘gangster thing’ [G n. (5)].

1994		Eble Campus Sl. Apr. 4: G-thing – pertaining to the life of a likeable, black hoodlum.
1998		Snoop Dogg ‘Still A G Thang’ 🎵 on Da Game is To Be Sold Not To Be Told [album] Top dollar with the gold flea collar / dippin’ in my blue Impala / Yeah it’s still a G thang.