It cannot open your dvd device, which likely means you have to make the symlink, easiest way I can think of without recieving information about your drives would be (assuming you only have a DVD drive):
ln -sf /dev/cdrom /dev/dvd
However it's improper to symlink to a symlink. So instead what you should do is find out where /dev/cdrom is pointing, do that by typing:
ls -l /dev/cdrom
And it will show you where /dev/cdrom is pointing to. At that point you can then symlink to the correct device with (as root):
ln -sf /dev/hdx /dev/dvd
Where /dev/hdx is the device you find from the above (ls -l) command.
If none of this makes any sense to you, feel free to post up what you get from typing:
ls -l /dev/cdrom
And also tell us a bit about your drive setup (Is your DVD your only drive, where is it at, the Primary or Secondary channel, Slave/Master and so on).
Also, to play a DVD the command is:
mplayer -dvd 1
gmplayer just brings up the gui
Cool