Hi. Just as a remembrance, when I first installed antiX 8 the video was badly configured (even monitor's refresh rates) and that was solved when nVidia driver was installed (I use an AGP video card over a mobo with a VIA onboard video that was automatically disabled).
At first I couldn't get gmplayer to work with videos (kept givin' the 'FATAL:Could not initialize video filters (-vf) or video output (-vo).' error) but since I have other o.s.' here, I didn't give much importance to it. Eventually I installed antiX base in a VirtualBox machine and downloaded/installed mplayer but couldn't get it to work too so uninstalled it, downloaded/installed VLC and all went well.
Some time ago I observed in a post of mine that while 'numlock' was set to 'on' on slim.conf, it wouldn't login with it enabled (and the same with 'hidecursor' set to 'true' not working, as I later on discovered).
Well, I found out that I had to change the video driver settings in gmplayer (from antiX default 'xvmc' to 'x11' for instance) for video to work (and it seems it's necessary to restart 'X' for it to remember the settings). Now, __{{emoticon}}__ Slim's settings are working.
Of course it may not have a cause/effect relation, but I read that gmplayer (and Brasero and maybe other apps too) have some bug that mess with Gnome localisations (like they extend their locale preference to - parts of - the DE).

Just saying in case it makes sense to a developer.