GWM is widely regarded as one of the most powerful feats, to the point of commonly being banned when DMs give out a free feat at 1st level. +10 damage is very powerful--and combos extremely well with the BA extra attack--at low levels.
One suggested fix I've seen is that it scales with level: The penalty you take is your proficiency bonus, in exchange for damage equaling 2x your proficiency bonus.
-2 to hit for +4 damage at levels 1-4
-3 for +6 damage at levels 5-8
etc
