Well, as long as U-Boot is working (means: You see a gp2x logo when switching on), do this:
1. Get 2.0.0 Firmware:
http://archive.gp2x.de/cgi-bin/cfiles.cgi?0,0,0,0,42,1472
2. Extract the archive onto an empty SD card (freshly formatted), except
for the gp2xboot.img
3. Put it into the gp2x (use fresh batteries!), press and hold START
+ SELECT while switching it on, until you see the "Upgrading Firmware"
screen.
4. This will only take a few seconds, then it will come up with the green
booting screen. After a few seconds, the upgrade should continue (if it
DOES NOT (wait about 30 seconds), then REPEAT step 3. It should continue
then
5. Wait a few minutes. The update will finish and the orange menu will
appear.
6. Now delete all the files from the SD-Card. Download FW 2.1.1.
http://archive.gp2x.de/cgi-bin/cfiles.cgi?0,0,0,0,42,1903
7. Extract the archive onto the SD Card as before.
8. Put it back in, press and hold START + SELECT while switching it on.
This will upgrade to FW 2.1.1
9. After it has booted to the menu, switch it off and on again (without
removing the card). During bootup, it will ask to continue the update. Do
this.
10. When the menu appears, the upgrade to 2.1.1 is finished. Congrats
Delete everything off your SD Card and enjoy your gp2x