This article describes how to remove the Gnome desktop environment from a Debian install.
Sometimes, unless you run the gauntlet of Expert Install (which is unnecessary if all you want is a general purpose, vanilla machine) then the Gnome desktop will be automatically installed from the installation DVD.
The following commands will remove it (and free up about 1GB disk space!):
# sudo apt-get autoremove gdm3
# sudo apt-get autoremove --purge gnome*