Sorry no, it is for Windows.
I might be wrong but it looks like company called Aspyr do ports to Mac. If i am not wrong they did port of some intresting strategy games like Crusaiders Kings 2, Europe Universalis 4, Hearst of Irons 4 and some children casual games like Civilization 5 and 6. So i do not see why they will not do Galactic Civilization 3 and all addons.
Maybe Stardock sould contact them and see by what agrements they will port Galactic Civilization 3.
I do not see anything for Stardock to lose but they can add Mac customers to existing PC castumers.
Afcause people who own both PC and Mac most likly will move to Mac. Hi hi hi.