I always have the same question for new Mac-compatible GOG releases...Mac-original versions are normally MUCH better and look way sharper than crappy VGA PC versions.
Despite its rave reviews, I am still struggling to play Fallout 2 on my iMac - it looks that terrible.
Stronghold 3 is also currently on sale on Steam. So dilemma is $6 for Stronghold HD on GOG or $7.50 for Stronghold 3 on Steam. The price is essentially the same... the question is which is the better game. Reviews for Stronghold 3 were bad - but that was mostly due to bugs which presumably have been worked out by now.
Stronghold 3 is also currently on sale on Steam. So dilemma is $6 for Stronghold HD on GOG or $7.50 for Stronghold 3 on Steam. The price is essentially the same... the question is which is the better game. Reviews for Stronghold 3 were bad - but that was mostly due to bugs which presumably have been worked out by now.
Stronghold Crusader HD, available on GOG also, is widely held as the best game in the series although the original Stronghold is said to be good too. In both cases these are setup to run in Wine for you using Wineskin.
There are no native older Mac games on GOG that I am aware of. All the old games are either running with Boxer or Wineskin. The only native Mac games they sell are current indies with Mac versions.
Personally, I would recommend you pick up either Stronghold HD or Stronghold Crusader HD on GOG and skip Stronghold 3 for the time being. If you love the games you can always get Stronghold 3 and hope for the best later on. It will be on sale for cheap again in the fall and winter.