I'm building a custom live Ubuntu Gnome 3.12.2 based, Only if I create a new user, set password and add him to sudo group,
setting by dpkg-reconfigure GDM ad default greeter, obtain a black screen and going to the first console appear "authentication failure" message.
otherwise, using dpkg-reconfigure and select the lightdm as default greeter all work fine: the greeter show the login screen and the user is able to authenticate.
Instead if I do not create any user, and set GDM ad default greeter, all work fine, but no login screen appear and the user goes directly to its desktop.
For build the iso I'm using the Ubuntu directive: https://help.ubuntu.com/community/LiveCDCustomization