If gdm is started as a daemon, changing inittab won't do a thing. You need to stop the daemon from starting.
I'm not sure how debian manages it's startup daemons, so I can't help you there. But you have a starting point now.
This should help you out: http://wiki.linuxquestions.org/wiki/Update-rc.d
Basically you use the command update-rc.d to manage startup services. So something like this:
# update-rc.d gdm remove