====== Start CentOS without UI ======

  systemctl set-default multi-user.target
  systemctl reboot

====== Start CentOS with UI ======

  systemctl set-default graphical.target
  systemctl reboot

====== Check current mode ======

  systemctl get-default

====== Stop GNOME UI ======

  systemctl stop gdm
  
Restart it back when needed:

  systemctl start gdm