Gaff vs. gimmick is a very interesting question. Because most writing in magic is done by people eager to share ideas, who are often not skilled writers, definitions and usage of magic jargon are often loose. Even careful writers can slip, through bad habits, when using these words. I'm guilty of this as well. The best distinctions I'm aware of were formulated by T. A. Waters in his excellent Encyclopedia of Magic and Magicians.
Thomas defined a gaff as a common object that has been physically altered in some secret way to bring about a magical goal; e.g., a stripper deck.
He defined a gimmick as a secret device never seen by the audience, which contributes to a magical effect; e.g., a reel.
To these two categories we should add a third: the fake or feke. Thomas defined these as visible devices, but with secret natures or uses that are unrecognized by the audience; e.g., thumb tips, mirror glasses. He judges feke as "an obsolete and affected spelling". I would differ with him on this point. I think this older spelling helps to differentiate the specific meaning of the word within magic's literature from the broader meanings understood by the public. I wish he were able to debate the point with me.
As with most things, there will be exceptions; items that can be plausibly put into more than one of these three classifications. But Thomas’s definitions provide excellent guidelines.