When you run gmplayer (GUI) version for the first time, you are going to get an error:
"New_Face failed. Maybe the font path is wrong. Please supply the text font file (~/.mplayer/subont.ttf)"
This is because the GUI needs to have a font associated with it for sub-titles.
So what I did was I opened up terminal window and did a search for fonts:
This will give you a long list of all of the ttf fonts in your system. I picked one called luxisr.ttf, found at:
/usr/share/X11/fonts/TTF/luxisr.ttf
I opened up Nautilus and navigated to that directory and copied luxisr.ttf (you can pick any font with a ttf extension) and pasted it into the
usr/local/share/mplayer/ directory.
I then renamed luxisr.ttf to subfont.ttf
then start up gmplayer again, it works.