Yeah the GC components only support IR or digital I/O (relays and digital input) control.
However, you shouldn’t need to use that component. A pair of the GC IP2SLs can be used as a transparent serial-Ethernet-serial bridge between a USB to serial adaptor (FTDI chipset seems to work) plugged in to the Pi and the amp at the other end. At least in theory. I have not tried it.
Try the default:
port: /dev/ttyUSB0