I installed the GNOME desktop environment through the Ubuntu Software Center on Ubuntu 14.04. It installed correctly, but the default login screen got changed to the GNOME login screen. Unity by itself still works as normal, and on the lock-screen, it still displays the default Unity one. How do I change the login screen from the GNOME one to the Unity one?
First I would check to see whether your system is running lightdm or gdm. You could check that by seeing which service is running, e.g. "service lightdm status" and "service gdm status"
If GDM is running then you want to switch back to lightdm. I believe you should be able to do that with this command. $ sudo dpkg-reconfigure -plow lightdm