Gimmick usually carries a negative tone. It's something that looks clever, attracts attention, but is really a deception in that it turns out not to be as useful as it seemed. Accessories are not necessarily gimmicks.
For example something like NES Zapper or other gun controllers e.g. GunCon are not gimmicks to me, as they did what they were meant to. It's not like they promised shooting, and then failed to live up to the promise of shooting.
The recent motion controllers (Wiimote, Move, Kinect) looked fun and promised revolutionary gaming, but then people realised the limitations, and compounded by a lack of really compelling uses in games, they did not live up to the promise. Something like SharpShooter for example looked awesome (you have aim, trigger, movement, reload, etc. nicely integrated) but in practice was a terrible way to play shooters. To more or less extent, these are gimmicks.
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