Anyone ever figure this out? I'm having the exact same problem. DPKG tells me I should stop GDM before installing a package. But I can't figure out how to stop it. I know the ctr alt F1 to switch to terminal command line, but that does not _stop_ gdm. It is still running and I can switch back by hitting alt F7. I need to actually kill the program and exit to command line. That's one of the frustrating things about Linux (Debian) that I have found. Why is there no exit key? I also tried going to the command line, switching to the /etc/init.d and typed gdm stop This did not work. It said "aborting... GDM already running."
Such simple things seem to have difficult answers with Linux.