You did not do anything wrong.
3790US can not use Sg/My 2.50 (so you get rebooting loop ., it's normal)
Nobody can flash 3790US to 3790Sg/My 2.50 till now.
For returning
1). Just flash your nuvi with the last rgn file via updater.exe (rgn file for 3790US to 3790Sg/My 2.30 you used.)
2). If you don't have that file., so just use original US gupdate.gcd saved to rgn file to flash your nuvi or make a rgn file to be Sg/My2.30 and flash again.
Gir Mismatch means you use resources.bin in different region from fw_all.bin
in this case your nuvi have Sg/My fw_all (and you flashed with non Sg/My resources.bin)
You can return to Sg/My2.30 as I said above.

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