In order to run gnome you must also install all the xorg/xserver stack as well as a lot of gnome pieces to get a functional desktop.
Normally you would just install ubuntu-desktop, which would install everything for you. But something in your question suggests what you want to do is install each package and see how it fits together to make a small install base. Run this:
apt-cache show ubuntu-desktop | grep Recommends:
And look through the results to find packages which you may need to install to get a gnome desktop working. Once you've got gdm and xorg installed it should just be a matter of:
sudo /etc/init.d/gdm start
And you can also make gdm start by default by adding it to the rc.d if it's not loading by default when you install it.