got it working
Never mind. Apparently, the reason why it wasn't working was that I was trying to run mplayer in the background (i.e., gmplayer &) and apparently either the gui or gnome didn't like that.
One more thing:
I compiled mplayer using the --with-xvidcore in ./configure, but the output showed this:
Checking for Xvid ...... no.
Looking in GMplayer preferences in the codecs tab, xvid isn't there, but win32, realplay, quicktime, and all the others are.