It's really hard to say - I've flown 35 segments this year, and will say that brought 5% of the time a bag will not fit in back and can make it's way into a bin over first class, and very rarely need to be gate checked. I board with group 2, so IDK if they are cutting off groups and forcing to gate check even if there is some bin space left, so that can happen to you.

I would ask myself, what is less stressful, sitting on the plane for 15m extra or moving people's bags on the side, and needing to get an FA to tell people to put their smaller items below?