Most of my servers are running RHEL 6.9, but some are running 7.4 and one has xwindows installed. Changing the runlevel is different now.
To check the current runlevel in RHEL 6.X:
# runlevel
To disable the GUI at boot-up in RHEL 6.x:
# vi /etc/inittab
Edit /etc/inittab and change the line id:5:initdefault to id:3:initdefault
Save the file and reboot to confirm.
To check the current runlevel in RHEL 7.X:
# systemctl get-default
To disable the GUI at boot-up in RHEL 7.x:
# systemctl set-default multi-user.target
Save the file and reboot to confirm.