Define Gimmick I suppose.
It's a gimmick in that as soon as Brady and/or BB retires, they won't be able to run it, at least not in the same way or as effectively. So yes, it's got a shelf live of effectiveness.
It's not a gimmick in the sense that it is a trick that can't last more than a few games, the Pats have already proved that wrong. It's a highly effective offensive system that relies on an incredible QB to run it and a very disciplined offense to execute it. Same is true for lots of other offenses.