I don't care for the word myself as it's typically thrown around just to mock something.
With that said, my personal idea of a gimmick is something that is added just for the sake of it in order to try & get attention.
In gaming, I don't consider the Wii Remote a gimmick. It actually did add something unique & different while certainly enhancing gameplay for many games. The DS having two screens & a touch screen also actually added to the games and can't be considered a gimmick. (To me)
The 3D in the 3DS is basically as gimmicky as you can get. Is it kind of cool? I'd say yeah. It's pretty neat. It adds practically nothing gameplay-wise though. It was an unnecessary feature added in an attempt to create some buzz. (In my opinion of course)
The GamePad feels like it was just thrown in there in an attempt to look new too. Fortunately, the Off-TV feature renders it actually worthwhile. It should've been an optional add-on but it's not a gimmick.
Gimmicks are everywhere though. Miley's Wrecking Ball video was an obvious one...it worked though.