gnome-session-flashback, there are parts of it that use parts of
Unity. So, not every part of
Unity should be removed.
LightDM is the Display Manger for Unity, where
GDM is the Display Manager for the
GNOME Desktop Environment.
GDM it is:
sudo apt-get install gdm
Note: If you already have GDM installed, run the following command to bring up the following setup:
sudo dpkg-reconfigure lightdm
During the install, you should see some screens like below:
Press Enter for OK and on the next screen, select the Display Manager you want:
Press Enter to select the Display Manager.
After it is selected, reboot your system for changes to take effect:
After the host is rebooted, you should be in a
gnome-shell from the terminal window, you should be able to start removing parts of Unity. I do not recommend removing all of Unity as there are still applications that rely on parts of Unity to work. One example I can think of in applications that rely other parts of desktops is
k3b is a very good disc burning application that relies on parts of the
KDesktop Environment, so parts of
KDE would be installed for
k3b to work.
It is safe to remove Unity itself:
sudo apt-get remove unity
it is also safe to remove LightDM:
sudo apt-get remove lightdm
it is also safe to remove the Ubuntu Desktop, which is the Unity Desktop itself.
sudo apt-get remove ubuntu-desktop
If you are removing any of the above and you see an application that it wants to remove that you are unsure of, do not remove it until you find out more information about the application.