It depends how it's used. If you have a duplicate card which the audience sees and believes to be the matching card then some purists might argue it's actually a feke (yes "feke", not "fake"). A feke is a piece of magical apparatus that is seen by the audience but appears to be normal in some way. A gimmick, strictly speaking, is a piece of apparatus which is not seen. A "gaff", these days, is an object which has been "gaffed", i.e., a normal object to which some secret alteration has been made. In the old days, "the gaff" was often used in the context of con tricks and games and meant whatever secret process led to the desired outcome, whether that was sleight-of-hand, a psychological subtlety or whatever, but we use it differently in modern magic.
Nowadays, we don't really hear the word "feke" much, its use having been superseded by "gaff" in a lot of contexts, and "gimmick" being used as a catch-all title for any magic paraphernalia. Nevertheless, I think that the three words don't mean exactly the same thing, so, for accuracy's sake I like to use them.