Setting Default Display Manager in Ubuntu

Ubuntu by default comes with a gdm display manager and I installed xdm to see how it looks like. xdm made the login screen not so great and from poweroff menu, reboot' and 'shutdown' disappeared. I decided to switch back to gdm, but couldn't figure out how to do it. I even uninstalled xdm, but this didn't change things and still xdm is the default display manager. I did bit of googling, but not quite getting what I am looking for.

Finally when I am browsing through /etc/X11 directory, I saw /etc/X11/gdm/default-display-manager file and changed the contents of that file to /usr/sbin/gdm and my display manager is back to gdm.