Greatsword + GWFS gives you + 1.533 damage per attack over duelling style with longsword and board for +2 AC.
As pointed out, it doesn't stack with GWM. It kind of stacks with Champion, since the damage increase benefits from crits, whereas duelling does not.
But all the fighting style are fairly weak-source. I think it would be a useful boost to chronically underpowered fighters to gate all Larian's special attacks behind an appropriate fighting style, making them unlimited use.