Some games like KOTOR 1 & 2 are missing their Mac versions on GOG yet Steam and other sites have the mac versions.

Other games missing the mac versions are Rayman Origins and jedi academy.

Games that I just can't find the Mac version at all are Unreal Tournament 1999 & 2004.

Most probably for legal reasons, not sure, we still don't have the Mac version of Bioshock on Steam either, for example, because the porting was done by Feral and they seem to prefer selling elsewhere (at least they added part 2 some weeks ago)

but UT2004 runs great, even on the latest OSX, if you combine the Mac demo version with the game files from here. I found instructions somewhere online, don't remember where though.
UT99 was only released for Mac OS 9 and really isn't usable anymore.

Many games run flawlessly with Wine though - Unreal, UT99, Deus Ex, Arcanum, would be nice if there were "official" versions to download directly...

the best choise to run games for windows that are not on mac is porting kit app that can download lots of wineskin wrapper to run games for windows on mac like native applications.

if you try to download porting kit
http://portingkit.com/en/
you can find inside this application both unreal tournament and unreal 2.

for games that on gog does not have a mac version you better buy where there is a mac version...
or you can make a wrapper with wineskin but is not always easy (i've made a wineskin wrapper for tales of monkey island). to make it you have to follow winehq instructions.

if don't want to learn how to make a wrapper you can still use a windows virtual machine or a bootcamp native windows.