(GDM 3 in Ubuntu 11.10)
Let's say you have both LightDM and GDM display managers installed on your computer (running Ubuntu, Linux Mint or any other Debian-based Linux distribution). LightDM is default but you want to switch to GDM - to do this, run the command below in a terminal:
sudo dpkg-reconfigure gdm
This will display something like this:
Select the display manager you want to use by default and hit enter. Then, restart your computer.
If GDM is installed, you can run the same command ("sudo dpkg-reconfigure gdm") to switch to any display manager, be it LightDM, MDM, KDM, Slim, GDM and so on. If GDM is not installed, replace "gdm" in the command above with one of the installed display managers (example: "sudo dpkg-reconfigure lightdm").