You don't have the display drivers set up correctly or you are running as the wrong user. I'm guessing you have nvidia.
Once that's working, run gmplayer and right-click on it somewhere, go to preferences and set up audio and video. Can use OpenGL on most systems if video drivers are installed correctly... click on a dvd.
You may need some dvd libraries. On fedora they are:
$ rpm -qa | grep livdvd
libdvdnav-0.1.10-3.fc5
libdvdcss-1.2.9-3.fc5.at
libdvdnav-0.1.10-3.fc5
libdvdread-0.9.6-3.fc5.at
libdvdread-0.9.6-3.fc5.at
This is how I set up my DVD to play automatically upon insert using xine. Can be adapted to use Mplayer.
http://comptune.com/forums/viewtopic.php?t=242