To disable GUI on boot, run:
sudo systemctl set-default multi-user.target
To enable GUI again issue the command:
sudo systemctl set-default graphical.target
to start Gui session on a system without a current GUI just execute:
sudo systemctl start gdm3.service
The same instructions that work for any Ubuntu 18.04 system
https://askubuntu.com/questions/1056363/how-to-disable-gui-on-boot-in-18-04-bionic-beaver