Thanks for your quick response,
I have gupdate.gcd in my device, it is same version as downloaded GCD, nuvi2xx7and2xx8C4_780.gcd, version 7.80. So I will use the one from my device.
Yes, I read post#4, I need to go through RGN file for the first time and patch it in pre-boot mode, I will try this tomorrow.
Is it possible to unpatch a patched firmware? just in case