Where does “ギャグ” come from?

ギャグ (Japanese) comes from English gag, from Middle English gaggen.

ギャグ (Japanese): joke

Definitions

Step	Language	Word	Meaning
1	English	gag	A device to restrain speech, such as a rag in the...
2	Middle English	gaggen	—

ギャグ — Etymology, Origin & Word History | Etymology Explorer