OS/X uses GCC 3.3 or 4.0 and as long as the code compiles under Linux it should work OK as a native application on the Mac. Linking the libraries will be the hardest part. I write and debug code on the PC in VC6, copy it to the iMac and build and debug it there and as long as you stay within ANSI C it works without too many issues. For stuff that works under Linux, running make on the same sources usually results in a working binary. OS/X Tiger is OpenBSD.
The IDE on Mac is XCode and with it you handle projects much like you would on the PC with VC++. The fun begins when you have to decide which frameworks need to be linked.
For a UB version of Quake 2 see
http://www.fruitz-of-dojo.de/php/home.php4As a server the mod DLL's must be built as Universal Binary shared libraries to run with the UB engine from the site above. You have zero chance of doing that if you don't have the source code for the mod.
For a user the easiest way to run a PC/Windows game on any iMac is to get Bootcamp and WindowsXP.
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