GDM killed on startup
Try this:
Ctrl+Alt+F1
sudo apt-get install -f -m --reinstall -u gdm
Meaning of the short options:
-f: fix broken
-m: fix missing
--reinstall: refresh archives to newest versions
Useful option:
-u: print out a list of all packages that are to be upgraded
Next:
sudo dpkg-reconfigure gdm
▶ select GDM instead LightDM
Finally:
sudo reboot
Enjoy!