As they say above, you need to SBF using RSD back to 2.2 froyo. From there, you can root, and then install liberty 2
That's the only way it will work. You cannot just flash over GB, since basically everything was changed when you updated to GB (radio image, bootloader, Firmware, etc.)
there are many great tutorials on the forum here that you can use
All you'll need is:
RSD
SBF file for froyo
Moto drivers pack
**just remember that when you do RSD it, make sure you have full battery, and you are comfortable doing it. Running out of battery or making a mistake could make your phone a nice shiny paperweight**
Members of the forum here, and the writers or the tutorials are not responsible if you do mess something up.
That being said, its an easy process, but you must be careful doing it
Sent from my GingerDroid 0.5 (Liberty)