A gimmick is a trick intended to attract attention. If you lure crowds to your bake sale by having cute football players handing out samples in front of your booth, that’s a gimmick.
A gimmick is often used by businesses to sell a product. As a sales gimmick, your local bank might offer free microwave ovens to people who open new savings accounts. A TV show may air a live episode as a gimmick to get people to watch the show. The word sometimes has a slightly negative connotation — there’s often some flashy trickery involved in a gimmick and it doesn’t always add value to what’s being sold.