Hiya!
TL;DR ...so maybe someone has already brought this up...
I voted "Overpowered". But I would have put an * after that. By itself, taken as-is, with everything else in the game being core only...no, not OP.
However, from what I've read and those I've talked to on the 'net, a campaign that is going to allow GWM/Sharpshooter is almost always also going to allow Multiclassing, point-guy, other Feats, Spells, Classes, Races and whatnot from other sources, etc. THIS is when GWM/SS becomes absolutely OP (moreso when players get together to create synergistic characters together). "I'm a human fighter with a 15 STR and GWM" isn't OP. "I'm a fighter/rogue winged tiefling with DEX 20 and Sharpshooter" is OP.