There seems to be a lot of confusion as to what a gag manga is sometimes, as WNL and Gintama come up occasionally, and they’re more plot-focused things.
Gag manga is what it says on the tin, a manga driven by gags instead of plot. So Kochikame, Dr Slump, Gag Manga Biyori? Gag-driven, gag manga. Gintama, Sket Dance, We Never Learn? Plot driven, so not gag manga.
Gag seems to thrive everywhere but Shonen Jump (for real no amount of talk about meme culture or youtube stops that being true, I don’t think that argument has a leg to stand on), and it’s hard to say why. It’s taken singular talents like Kyosuke Usuta and Ryo Nakama to make gag manga work in 21st century Jump, and the rare 33% hit rate of Kōji Ōishi, scraping in with Inumaru Dashi (tellingly, the second Lycopene moved out of Jump it started doing better).
There’s just something about Shonen Jump’s audience nowadays. Miserable lot, I guess.