Update ::
It works fine now on my 3790.
At first it didn't work because I renamed Mitsubishi.gpi to other name for displaying 2 Mitsu icons.
When I deleted one of Mitsu.gpi --> Volvo.gpi work now.

At first :: I'm not sure where I should place .gpi within Nuvi so I installed in sdcard\poi\volvo.gpi
Then install to internal memory by firmware prompted "Do u want to install" --> Yes
I found that .gpi file was place in NUVI\POI\volvo.gpi