I am not sure how accurately you described what you installed, but what you need is gnome
. AFAIK gdm is part of gnome, but gnome is not part of gdm, that is, with installing gdm you won't pull in a desktop.
You can also try to run:
update-rc.d gdm defaults
to add gdm to the startup programs.
Are you sure that you want gnome and not KDE? Wars have been started for less than preference for either gnome or KDE, but if you have formed yourself an opinion and you decided for KDE, install kde, kdm, kdesktop
and that should pull in about 500 MB of packages, install a full GUI environment and eventually start up in a GUI.
Sorry about the confusion with runlevels in Debian. It is not according to what is standard in *nix. Debian should not have tampered with runlevels in the way they did, although they must have had their reasons.