Do you mean gnome as a window manager ? don't know if is possible without installing an XWindow server first.
You've got an X11 server on your regular MacOSXsystem DVD in the optionnal packages section
You'll also have to install XCode 3.1 (you can get it for free from the site
Apple Developer Connection - Developer Tools and Technologies - Download Xcode after your registration to the ADC which is free also)
After that you must get the installer for one of the two main GNU packages ditributor :
MacPorts or
Fink.
Whether one or another is a matter of tastes. Both install their packages in their own directory. You will have to adapt your bash profile in order to access the binaries. After that through CLI or IHM you can get in the download section of the Apple site, you may download and recompile nearly any GNU stuff you want, even Scribus, Gimp, etc.