I've seen a few people having trouble connecting because of a simple ifconfig error:
The ip in your GP2X settings menu must be different from the one you use to create the usb0 device under linux.
Basically, if you run "ifconfig usb0 192.168.5.1 up" with the ip set to 192.168.5.1 in the gp2x...
A few links to point out:
http://en.wikipedia.org/wiki/Matroska
http://en.wikipedia.org/wiki/OGM
http://en.wikipedia.org/wiki/Comparison_of_container_formats
From http://en.wikipedia.org/wiki/OGM:
Basically, OGM is just a quick and dirty hack of the OGG audio format that supports OGG...