Can someone please clarify if MVP/Golds or BankOfAmerica card holders are exempt from the gate-check fee or not if bag doesn't fit sizer (since technically they do get free checked bag).

Alaska should really have a reasonable enforcement here (similar to other airlines). Reasonable means that bags which are obviously several inches too big should be denied, but half/one inch over here and there or the wheels causing issues shouldn't be a reason for denial since those can fit in the cabin hold fine. And specially for horizon a-la-cart stuff there really is no need for any draconian enforcement at all since those don't go on the plane anyways.