OS X has pretty easy access to a GCC development environment installed with XCode (an IDE and development suite for making OS X and iPhone applications). Installing XCode is straightforward, you can get it from either your OS X install disc that came with the computer, or from Apples site.
To install from the disc, put the disc in the computer and select Optional Installs. In the new window, launch the XCode installer package. Follow the directions on the screen, the defaults should be fine for use with RVM.