GDM is the GNOME Display Manager, a graphical login program; Ubuntu and Xubuntu use GDM by default.
This howto does not apply for Kubuntu, which uses KDM, the KDE Display manager.
How to auto-login into your Desktop environment:
Select the Security Tab,
Flag the “Enable Automatic Login”,
Select your name from the drop down menu,
From now on your pc wil auto login to your username.
How to set up a new theme for GDM:
First make sure to have the universe repositories set up
sudo apt-get install gdm-themes
When gdmsetup opens, you see a list of themes you can select from.
Select your preferred theme and close gdmsetup, done.
This step is not needed if you have downloaded a theme from art.gnome.org and wish to add it.
Then you select, after opening gdmsetup, add, then select the package you downloaded, click install and gdmsetup will install the newly downloaded theme for you. (Dont extract the downloaded package yourself!)
Of course this also goes for all other distros that use gdm exept for the “sudo apt-get install gdm-themes” bit.