Green’s Dictionary of Slang

gam on v.

[gammon v. (4)]

(Irish) to pretend, to make out that.

[Ire](con. 1940s) B. Behan Borstal Boy 164: They gammed on not to take them seriously.
[Ire]T. Murphy Whistle in the Dark Act I: Them are the ones that gam on not to know you when they meet you.