By default, root can not login to gnome in debian or ubuntu. In case, there’s another way to enable root login to gnome permanently. In a console just edit file :

$ sudo gedit /etc/gdm/gdm.conf

In the [security] section of the file, change “AllowRoot=false“  to “AllowRoot=true“. Or you can add following line :

AllowRoot=true

Save it and you need to reboot or restart the gdm to find out.

Or you can do this by gnome, go to menu “System -> Administration -> Login Window” :

Go to “Security” tab and just check “Allow local system administrator login“.

Close dan reboot.