Open your Te rminal from Applications -> System Tools -> Terminal

———–Now Login as a root from your terminal

————Now go to your :– cd /etc/pam.d/

————-Then first take a backup of gdm file

***********************cp gdm gdm.bkp

—————Find and Comment or remove this line into your gdm file
—————auth required pam_succeed_if.so user != root quiet

Save & Exit From that File.

—————–Then first take a backup of gdm-password file

***************cp gdm-password gdm-password.bkp

——————–Find and Comment or remove this line into your gdm file
——————–auth required pam_succeed_if.so user != root quiet

Save & Exit from File.