can't install gnome

I tried to install gnome and I got the following:

Could not mark all packages for installation or upgrade

The following packages have unresolvable dependencies.
Make sure that all required repositories are added and enabled in the preferences.

gnome:
Depends: gnome-desktop-environment but it is not going to be installed

I have testing and unstable checked in the repositories but nothing else.

Any help would be greatly appreciated.