What mainline OS and firmware revision are you running on your 770? OS 2006 or are you running 2007 Hacker Edition? If you are running 2006, I think you might need to install the 3rd party Maemo Bluetooth plugin from http://770.fs-security.com/maemo-bt-plugin/ and use it to pair the GPS to your 770 and then you also need to go into Maemo Mapper and give it your bluetooth gps address. Make sure to get the correct version of Maemo Mapper for your OS here:

https://garage.maemo.org/projects/maemo-mapper/

I use the M-1000 with my N800 and Maemo Mapper and just love it.