Green’s Dictionary of Slang

1. (US black) a trumpet player.

1944		Cab Calloway New Hepsters Dict. in Calloway (1976) 255: gabriels: trumpet players.
1944		‘Jiver’s Bible’ in D. Burley Orig. Hbk of Harlem Jive.

2. (UK prison) the chapel organist.

1950

P. Tempest Lag’s Lex. 89: Gabriel [...] Nickname given to the chapel organist.