Here is a working procedure to get gp2x usbnet working with a linux box :
Download g_ether from http://gp2x.sector808.org/g_ether.o
download the 2.0 firmware, unzip it.
then, unzip gp2xfs.tar.gz, extract g_ether, delete it, replace it with the new one, re-append that file to the archive, then...