A gimmick is:
A good example of a magician's gimmick can be seen in the movie "The Prestige". There is a bird in a cage that two audience members place their hands on. The magician triggers a device on his back that folds the cage and retracts it up his sleeve. There are several more, but the ending gimmick developed by Tesla braches off into science fiction territory.
For illegal gamblers, a gimmick can be a device that helps them with counting cards or to calculate odds. Loaded dice is another gimmick.
An attention-gathering scheme or strategy to make something more appealing
If you look back on any of the old commercials you remember from years ago, you're more likely to remember a gimmick or a jingle than anything. For the folks in Denver, the old American Furniture Warehouse ads used live tigers. For New Yorkers, there was the old Crazy Eddie commercials that had a loud, manic-sounding salesman shouting about how the deals were insane.
A hidden problem purposely concealed that negates a good or useful item.
They say if something is too good to be true, it's a scam. That's the gimmick -- there's something in the small print that kills the deal or the new stereo you're thinking about buying out of the back of an unmarked white van is filled with junk components or even bricks to give it weight. When the new PlayStation 2 came out, people were selling a new box for hundreds of dollars when nobody else could get their hands on the units. It turned out they were only selling the box the PlayStation 2 came in, not the unit itself.
Iron Noder 2017