If you have one day installed any other Display Manager to your Ubuntu, set it as default DM and do not know how to revert the original DM > GNOME, you should execute below command which will solve your problem
sudo dpkg-reconfigure gdm
I experienced this caused I installed KDE to my system
February 3rd, 2009 - 2:33 am
Thanks a lot! You saved my day, today. I tried to change gnome per Kde and when decided to recover gnome and erase kde I didn’t put gnome again as default display manager.
You saved my day!
Thanks again,
Nyap