Hi!
Excellent! The extra -1 argument was the final culprit. Thanks a lot - I'd never have been able to figure it out myself. I made a PR for the ArduinoGateway sketch.
Like you suggested, the messages I quoted indeed weren't the ones written to the serial port. And I believe I earlier ran into issue of Due getting stuck due to unread data. Pretty weird. I think I'll scavenge the Teensy I have and repurpose that into a gateway...
I was still able to get a couple of lines of "Update operating mode" resulting in a timeout on the NMEA Reader Log tab. But the reason for that is by now pretty self-evident.
Again, thanks a lot for your help.