GNOME is an X window system client. As such, it requires an X server to which it can connect. Installing the GNOME packages in Debian does not automatically install an X server for you, much to the surprise of many people. This is because the X server does not necessarily have to run on the same machine as the client, and you might in theory have a GNOME application server which has no X server of its own, but rather, acts as a workhorse for a fleet of thin X terminals.
Most people who will be reading this document, however, will just have one workstation with X and GNOME on it. So, before you can run GNOME, you must install X, by following the advice on the ConfiguringX page.
After that, assuming you run sarge, you can simply apt-get install gnome to bring in the GNOME software, including the gdm display manager (graphical login). If you simply want to login with gdm and let it run GNOME for you, you're done.
If you'd prefer to start X without gdm, then simply apt-get remove gdm and make sure you have exec gnome-session in your ~/.xsession file.