Hello, well it looks like RPI 3 uses ttyS0, a miniUART, but I'm not sure it has enough baudrate, it is intended for console use. I didn't test it yet... The ttyAMA0 is used for the bluetooth, there might be ways to use it...
Hello List, Matlo, in https://gimx.fr/wiki/index.php?title=RPi , chapter "USB adapter using the on-board UART interface/Sotware adjustments", shouldn't there be an entry stating we should use -p /dev/ttyAMA0 when starting Gimx... Sorry if I'm pointing the obvious, but hey I had to look for...