Green’s Dictionary of Slang

gag adj.

[gag n. (1)]

joking.

[US]Da Bomb [Internet] 12: Gag: Joke. They gave him a gag gift.
[US]C. Cook Robbers (2001) 59: Giggling when she said it, like it [i.e. a knife] was a gag gift.
[US]E. Weiner Drop Dead, My Lovely (2005) 69: One of those life-sized photo standups of celebs you pose with for a gag snapshot.