No, a gate check is different. A lot of casual flyers don't want to "lug their bag all the way to the gate", including through non-precheck TSA, esp if they get the airport early and want to do some shopping or eat some food. Once they are at the podium and the flight is boarding, the marginal hassle of getting the bag on the plane is small (and less, maybe, than waiting at baggage claim on the other end).

I offer as evidence that the overhead bin space was usually not as crowded before the majors starting charging for bags (2008). Yes, loads were lighter too, but I think there was a natural equilibrium where enough people checked bags that it wasn't such a crisis like it is now. Add a $25 fee for the half the plane that is likely to consider checking, and boom: big problem.