Testing GPS

[wibble]

Would it be correct to assume, then, that we're getting GNSS messages via NMEA strings over UART, and gpsd is just parsing those?

That's correct - NMEA received via a one-way USB serial interface. Config, enabling and disabling is done via AT commands over one of the other USB serial interfaces. i assume ubports is doing the same as pmOS by giving gpsd commands to enable gps when the first client connects, and disable it when the last one disconnects.