I think we’re talking about two different things. I’m not trying to maximize melee DPR or assume a steady supply of heavy magic weapons. I’m looking at how to give a STR‑primary Fighter a reliable ranged mode with minimal investment.
In this context, GWM is actually very low‑risk:
- The +PB damage applies to longbow attacks
- The bonus‑action attack on crit/kill still works with non‑heavy melee weapons
- The STR ASIs still benefit any melee weapon you end up using
So even in the worst case where there are no magical heavy weapons and you use a magical non-heavy weapon, the feat still contributes meaningfully to the character.
The upside (when you do get to use a heavy weapon) is huge, and the downside is pretty mild which is why I see it as a high‑reward, low‑risk pick for this chassis.