Green’s Dictionary of Slang

(US) a chatterer or complainer; a braggart.

1942		Z.N. Hurston ‘Story in Harlem Sl.’ in Novels and Stories (1995) 1009: Gum beater: a blowhard, a braggart.
1944		D. Burley Orig. Hbk of Harlem Jive 123: This is your ace gumbeater, Hentracking like Gabriel Heather.
1946		Mezzrow & Wolfe Really the Blues [dedication] To the sweet-talkers, the gumbeaters, the high-jivers, out of the gallion for good and never going to take low again. (You got to make it, daddy.).
1955		B. Schulberg On the Waterfront (1964) 98: Christ, what a gum-beater.
1995		B. Broeg Memories of a Hall of Fame Sportswriter 167: Personally, all I’d ever heard was a relentless lobby gum-beater, Harry Walker, quoting brother Dixie of the Dodgers.