I just tried to switch to gnome after using Unity for so long. So I installed gnome from Ubuntu Software Center. It was installed perfectly. But after restart, I couldn’t enter into the login screen because of a black screen.
The culprit was the gdm(Gnome Display Manager) not starting. I was not looking to make this working, may be later I find something, but at that time I wanted to login to my PC anyhow. So to recover from this, I booted into recovery mode as read write enabled.
mount -o remount,rw /
This will give you the option to select a display manager, choose lightdm. This is the default one used in Unity.
After that we need to remove gdm
apt-get purge gdm
After that, reboot and you will be back into your Unity login screen.