Green’s Dictionary of Slang

1. a gossip, a loudmouth; thus gate-mouthed, loudmouthed.

1926		New Orleans Wanderers [song title] Gate Mouth.
1944		D. Burley N.Y. Amsterdam News 9 Sept. 12A: Gatemotuh Moore, the Memphis-via-Washington blues shouter.
1965		(con. 1920s) Carmichael & Longstreet Sometimes I Wonder 203: Lil Hardin, the piano player, took the gate-mouthed Louis under care.
1996		Eble Sl. and Sociability 83: The area of inter-personal relationships includes gatemouth ‘a person who knows and tells everyone else’s business’.

2. a person, a man.

1941		D. Burley N.Y. Amsterdam Star-News 19 July 13: I latches onto a Gatemouth with his hair blowin’ back.
1944		D. Burley Orig. Hbk of Harlem Jive 64: I [...] latches onto a gatemouth with his hair blowing back.