Note:
You will need the root password for this set of commands.
Step1:
Login to a user account.
Step2:
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
Step3:
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.
BEFORE
auth required pam_succeed_if.so user != root quiet_success
AFTER
#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.
Reboot.
I hope this is helping some of you, because root is not allowed to login on a fresh Debian 6 installation.
credits:
http://www.pathin.org/tutorials/debian-6-howto-enable-gnome-root-login-on-debian-6-squeeze/