The setup utility that comes in Ubuntu Karmic is missing most of the older GDM setup utility's functionality. If you're tired of not being able to configure many of these settings, two community members have come to your rescue. Garth Johnson and Nick Glynn have developed a great tool called gdm2setup which put all of the functionality back in the hands of the users through a familiar and simple GUI.
Grab it here or try it out from their PPA!
Source: Robert Ancell