Green’s Dictionary of Slang

gabbo n.

also gabo

[colloq. gab, to chatter + -o sfx (7)]

(US) a chatterer.

1931		G. Milburn ‘Convicts’ Jargon’ in AS VI:6 438: gabbo, n. A talkative person.
1950		Goldin et al. DAUL 76/1: Gabo. One who talks too much.