I have installed ubuntu-unity-desktop package from my ubuntu 18.04LTS installation and I wish to change into unity or any other desktop environment I have installed in it.
But on the login screen I find no option to select between gnome and unity. Do you know how I will enable this option into GDM?
Did you reboot the computer? The change to take effect needs your computer to be rebooted afterwards, a gear icon will be shown and then you can select the appropriate "GUI" you desire.