Rename gupdate_Patched.gcd to gupdate.gcd before copying to Montana!

You must have on device SW lower then version 4.20! If no you must install previous version on device. Copy gupdate.gcd with previous original SW to device only. Reboot for installing of old SW and than use new patched SW.
The same way for come back to the unpatched version - you need to re-install previous original version of SW and then install 4.20 or simply install the latest original SW (above 4.20) if it is present.

Warning! All come back will clear your data - make back up for restore!!! Basecamp suits fine for this purpose...