Hwid within garmindevice.xml was "read" from string (Unicode) in fw_all.bin located at hex: 7F5BD6 (Let say about Sg/My 2.30 fw_all.bin).
This hwid within garmindevice.xml can be wrong if the device used to flash with mod .rgn made from RGN_Tool v0.44 0r 0.50.
You can see what I said just you use 0.44 to convert nuvi. This issue is fixed in v0.60 already.

If you are sure about .gcd file you uploaded., you have to ensure about what really hwid your nuvi is.
To flash nuvi with wrong hwid boot.bin will brick your unit.