Green’s Dictionary of Slang

gag adj.

joking.

1997–2000		Da Bomb 🌐 12: Gag: Joke. They gave him a gag gift.
2000		C. Cook Robbers (2001) 59: Giggling when she said it, like it [i.e. a knife] was a gag gift.
2004		E. Weiner Drop Dead, My Lovely (2005) 69: One of those life-sized photo standups of celebs you pose with for a gag snapshot.