Green’s Dictionary of Slang

(US Und.) to act as a spy for a criminal gang.

1916		A. Stringer Door of Dread 60: How d’yuh know he ain’t gay-cattin’ for Keudell right along?
1921		P. & T. Casey Gay-cat 204: A lazy, easy life wot beats [...] gay-cattin’ for a bunch o’ circus sharpers!