You will need the root password for this set of commands.
Login to a user account.
Open a terminal window and enter the following. This will open a nano tex editor window in the terminal.
su root nano /etc/pam.d/gdm3
Now open the file /etc/pam.d/gdm3 and comment out the line, see the example if you do not know how to comment out a line.
auth required pam_succeed_if.so user != root quiet_success
#auth required pam_succeed_if.so user != root quiet_success
Press Crtl-X to close the file.
Press Y to save your changes.
Press enter to save changes to the same file name.
I hope this is helping some of you, because root is not allowed to login on a fresh Debian 6 installation.