Mother of all detours; the USB port on the Garmin GND10 isn’t NMEA2000, it is something called Nexus FDX. That took a couple of weeks to figure out, hunting for NMEA2000 protocol headers. Always check your assumptions.
Anyway, I’ve written a decoder for it now which can be found on https://github.com/lkarsten/GND10read . It decodes GPS position, speed, wind angle and strength. Output can be a series of json encoded dictionaries, or NMEA0183 if you prefer.
I’m pretty sure I’ve gotten the header format of the FDX packets wrong, but it seems to work. The git repo above has my notes and some dump files, if anyone has a proper FDX network and want to try it out.
Use case is simple. I connect my laptop to the GND10 USB port before I go out sailing, and I get all the sensor data into OpenCPN. I can also log it to file, if I want to compute my own polars or do endless post-regatta analysis.