Stop GDM from starting at boot

You need to change your run level or change the services that is executed
for that runlevel, see below.

• Runlevel 0 is halt.
• Runlevel 1 is single-user.
• Runlevels 2-5 are multi-user.
• Runlevel 6 is reboot.

First open /etc/inittab and look for the default runlevel.

# The default runlevel.
id:2:initdefault:

It’s 2 in this case.
Next go to ‘/etc/rc2.d’ there you will see a symbolic link to gdm,
usually S99gdm->../init.d/gdm
Remove S99gdm and you system will not boot into X