Hmmm...at what point does somebody's voice become a gimmick? I'd not even say that about Arnold Stang or Don Messick (although the latter's wheezy dog-laugh, yeah, that's a gimmick. but it's not his voice).
Now those rotating bow ties that the Spike Jones band wore, that was a gimmick (and a nifty one, at that...). Jackie McLean's tone...that's just another way to play the alto. Blow an alto like a tenor and that's where it starts going. Refine it to where you hear it, and there it is.
Time, place, people, intent, all that factors into it. "Gimmick" seems to me a bit dismissive of all of that. I've know alto players over the years who really, really don't like McLean, but it's always because of something like "he blows the alto like a tenor". To which I say, hey, too bad. It's a horn, period. Blow it like you feel it. Same thing with the poeple who bitched about Warne MArsh sounding like an alto instead of a tenor (yeah, if that sounds like an alto to you...please don't drive while blind, I don't trust your hearing);.
But that comes down to esthetics, and the various prissy finickries therein (damn alto players...). This is the very first time I've ever heard him accused of gimmickry.