While writing comment on another site, I accidentally came up with what I think is a pretty good definition of "gimmick", since the internet tends to throw the term around with wild abandon.

A storyteller that likes getting paid should do things that make the audience want to come back for more. A gimmick is when you do something that win in the short term, but is ultimately burning your audience as they tire of it.

The linked article talks about one of JJ Abram's go-to gimmicks. It's a good one that has carried him quite far, but it's ultimately a gimmick as people figure out that the answer to the questions that his stories pose is "There is no answer, and in fact, whatever you think it is, it's not that."

That latter clause in particular is not only gimmicky, but kinda a jerk move. When watching an ongoing series, the Internet as a whole will propose every sensible possibility, every crazy possibility, every wish-fullfillment possibility, and every out-right stupid possibility. If what is truly going to happen isn't in the set of things the Internet is guessing, you'd better have a really good reason, or otherwise, that is a strong signal that you have screwed up and written a dumb story, if nobody at all could guess what was coming next. You don't want 100% of the audience to be able to guess, but you also don't want to be writing stories where 0% of the audience guessed.

Motion controls for games are often referred to as "gimmicks", but using this metric we can distinguish some differences. Motion controls that are replacements for button presses are gimmicks. Nifty at first, but eventually the game instinctive notices the lower precision and higher latency of the motion control, and begins to hate it. Motion controls that use the continuous, physical nature of motion controls to do something that requires it is not a gimmick. I find even years later, using them to aim, or steer cars in Mario Kart, is still a superior control method, sometimes even the best commonly-available method.

The best test for a gimmick is time. If you want to go back to it a couple of years later, it's probably not a gimmick. If the fad rises, burns across the landscape, and then fades with hardly a trace (found footage movies, for instance), it's a gimmick.