There's this great OS by Microsoft called Windows that you can install on your PC. For gaming, it performs much better than OS X.
The best thing about it is that it's actually made for PCs so there's not much hacking involved, you don't need to mess around with boot loaders, perl scripts and patched kernel extensions.
If you are running OS X on an actual Mac, install Windows on it and play the Windows version. Macs can dual boot and run Windows natively.
It's great if you like games but don't want to "pollute" your precious production environment (ie, your expensive MacPro used in a recording studio or for graphic design and video editing)
Prepare to be disappointed, unless you own a MacPro with a fast Kepler card/ATI equivalent or better, there's no way you'll be able to play GTA 4, much less 5 on your Mac.