How do I downgrade Android 2.1 to 1.6 on a Garminfone?
not sure about the root process for Garminfone, but if you have access to your bootloader (usually termal like with a list of options) just put a 1.6 stock rom on your sd card and flash it.

I tried googleing "stock 1.6 garminfone rom" with no success, probably because of what GAThrawn said, but someone over at xda-developers might have something if want to go check out there forums.