how did you start it? if you just ran gdm, you can kill it: try "ps aux" for the pid (you may want to kill X separately as well if killing gdm doesn't do that)
if it starts at boot and you don't want it to, edit the runlevel line in /etc/inittab
if you just want to go to a console temporarily, try "ctrl-alt-F#", where #=1-6
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