Today I just changing my desktop environtment from XFCE to Gnome 3. However GDM service always failed to be enabled. Everytime I ran
sudo systemctl enable gdm, it always return
Failed to execute operation: File exists.
This caused by another display manager still enabled, but I don’t know which one. Fortunately, running
sudo systemctl enable gdm -f will force GDM to be enabled and another display manager will be disabled.
Removed symlink /etc/systemd/system/display-manager.service.
Created symlink from /etc/systemd/system/display-manager.service to /usr/lib/systemd/system/gdm.service.