Didn’t check with GWM as I didn’t select it. But according to what you say, are you sure it’s not intended? Damage for polearm bonus attack is fixed as per description. While primary attack value applies to the bonus attack roll, including the -5 penalty if GWM is active. Not a great deal.
Tabletop text doesn’t seems to resolve this, unless there are FAQs going in deeper details for this kind of combination.
I’m just reading text in a certain way, not saying it’s the right interpretation.
