A search would have given you all the answers you need, as the subject crops up here quite often.
But now im here:
As far as im aware, Gears is console only and it looks like Epic have no intention of bringing it to PC's, never mind Macs.
UT3 for Macs is known to be in the works however. But its been in the works for sometime now. Meanwhile, online PC play is dying off through lack of interest

(Persistent bugs, and small selection of now tired maps isn't helping). Hence the PC version being sold in the bargain basement bin of stores already, and has been for a while now.
There is some chatter on Epics (diabolical) forums about a UT3 expansion coming at some point, and I wonder if Mac and Linux versions are being held back for this. So that all platforms get a more or less simultaneous launch. That would also give Epic an excuse to be able to sell all versions at the same price again. Understandably people would complain if the PC version was being sold at say $10, yet the otherwise identical Mac one at $50+
Don't ask when the Mac version will come however, as no one knows. In the mean time.. If you have an Intel Mac, then UT3 runs fairly decently via Bootcamp