How do I switch from the default display manager (Lightdm) to the older display manager (GDM)?
Switching from LightDM to the "older" GDM is very easy, and need not even involve opening a terminal.
Easily switch to GDM with the Software Centre
Switch to GDM via the terminal
Gnome-display-manager is not broken in 12.04 (or at least it works fine for me), There is no need to reboot the machine. You ran the right commands, but maybe try it this way, also might want to
after this you will need to press alt+f2 and login from command line.
Then you will be able to start