Merriam-Webster defines gimmick as:
a: a mechanical device for secretly and dishonestly controlling gambling apparatus
b: an ingenious or novel mechanical device
a: an important feature that is not immediately apparent
b: an ingenious and usually new scheme or angle
c: a trick or device used to attract business or attention <a marketing gimmick>
While video games are products, and therefore subject to marketing gimmicks, the way the word is usually used in competitive gaming is a mix of most of the other definitions listed with a few special twists. Standard issue for words being co-opted into a specialized lexicon.
What we’re looking at is more like this:
a tactic designed to exploit lack of knowledge or preparedness in the opponent with little use against opponents who have done their homework
It’s important to keep the second half of that definition in mind. Obviously every tactic employed in a competitive game is supposed to exploit an opponent’s knowledge and ability to prepare (either beforehand, or with reactions in the moment), but what sets gimmicks apart is that they really only succeed against an opponent doesn’t know how they’re supposed to defend against them.
Common examples of this type of gimmick include most unblockable setups and many tech traps, resets, and frame traps (and in the case of games with shakeable stuns, entire combos) in fighting games; certain styles of rushes or timing attacks in Starcraft games, some ability or lane combinations in DotA, and I guess there are plenty of strange mechanical and map inconsistencies in FPS games that would qualify. I’m mostly concerned with fighting games and DotA here since that’s where my most recent knowledge comes from, and also where I find the idea is most often used.