A gimmick is an ingenious or novel device, scheme, or stratagem, esp. one designed to attract attention or increase appeal.
Really? Let's just see what the dictionary has to say about that!
...Alright, never mind. Dictionaries aren't very reliable anyway, so let's check what Wikipedia, the infallible wellspring of knowledge, defines the term as:
a gimmick is a quirky feature that distinguishes a product or service without adding any obvious function or value. Thus, a gimmick sells solely on the basis of distinctiveness and may not appeal to the more savvy or shrewd customer.
SUCK IT
...But seriously, in this context, "innovation" and "gimmick" don't overlap. In gaming, gimmicks are employed
in place of valuable creative elements. Why put thought into an inventive new game when one can just stuff popular characters into a cookie-cutter formula and watch fans clamour over it, right?