Indicator whether or not it is for Mac

You know, that little mac symbol

If you look on the bottom of the Steam game page, it'll show some system requirements. The game will run on Mac if there's Mac system requirements.

Doesn´t seem to explain itself well enough ;) I guess you mean here on steamgifts, and not on steam itself, right? Guess it would be nice for you if something like that gets added officially, but until then you might help yourself by using addons (if you can use em on mac).. you could use the filter-function to filter out every game thats not on mac by hand... just click the "add to filter" button at a giveaway with such a game once and you´ll never see it again ^^ Might take a while until you filtered out most of the games though.