I suggest that you read the definitions and example sentences in an advanced learner's dictionary (e.g. Cambridge, Collins). We can do no better than them. Then, write your own sentences and/or explain your own understanding of the differences. Then, we can give you useful feedback.

Gimmick = when someone sells you something for a lot of money because it is of much value TO YOU, for example: a baseball card with a baseballer's signature and/or photo is a gimmick. "Stunt" - usually means "marketing stunt", for example: you buy 2 boxes of Cola and we'll give you a gimmick, such as a baseball card, that's a marketing stunt. "trick", also means in given context "marketing trick": a way to get other people (consumers, customers, clients) to do what you want them to do (to buy more Cola) using gimmicks and stunts. But stunt can refer to a whole marketing campaign, while trick is a particular incidence of a stunt, something a lot shorter and of a briefer duration. There are many tricks to make a stunt work.