I got curious and tried the GeeXboX unsupported build. It sort of works, I can't get audio, but some video files will play, it may only play audio over HDMI.
I had an 8GB SD card with boot and rootfs partitions set up from other Pandaboard trials, so it was easy.
1) Erase the contents of the SD card boot partition
2) Download and unpack:
http://download.geexbox.org/unsupported/...ard.tar.xz
3) cd geexbox-flat-devel-20140628-r894b43a.pandaboard and copy the contents of the boot directory to the SD card boot partition, I used Nautilus drag and drop.
4) format the SD card rootfs ext4 and mount it, I made the label be Pandaboard this time.
5) as root cd to where you have the downloaded *.xz file:
tar xfp geexbox-devel-20140628-r894b43a.pandaboard.tar.xz -C /media/Pandaboard
6) Unfortunately this creates /media/Pandaboard/geexbox-flat-devel-20140628-r894b43a.pandaboard/ so you need to move everything in this folder up one level. I did gksu nautilus and navigated to inside /media/Pandaboard/geexbox-flat-devel-20140628-r894b43a.pandaboard, enabled show hidden files, did Ctrl-A to select everything, Ctrl-X to cut, and navigated up to /media/Pandaboard and did Ctrl-V to paste.
7) Unmount the SD card and boot it in the Pandaboard.
Good Luck!
Maybe poking around this system will give you clues as to the bits you'll need to build on Gentoo. This does seem to be the best working GUI I've seen yet on my Pandaboard ES.