This is a vintage transceiver without an accessory port for digital modes. With that in mind we’ll be interfacing through the mic connector. What makes the homebrew build easier is the presence of the received audio on the mic connector:
GX16-8 pin 1 (MIC INPUT) → 3.5mm TRRS ring 1 (RIG_AFIN)
GX16-8 pin 5 (PTT) → 3.5mm TRRS ring 2 (RIG_PTT)
GX16-8 pins 6 & 7 (GND) → 3.5mm sleeve (GND)
GX16-8 pins 8 (AF OUTPUT) → 3.5mm tip (RIG_AFOUT)