Can't switch to gdm3 using sudo dpkg-reconfigure gdm3

re: sudo dpkg-reconfigure gdm3 error message.

Just reboot and you should be running gdm3.

The error that you're receiving just means that gdm3 wasn't already running, so it couldn't reload it.

Use this sudo dpkg-reconfigure lightdm in place of sudo dpkg-reconfigure gdm3 this and then choose gdm3.