Well that rules out the SB Z optical in being at fault, if you can get sound by connecting a PS4. Still could be something to do with that, not so much a fault, just some incompatibility somewhere.
You could try a smartphone or anything else that has analogue audio output, instead of the GSX. Can't really see why the ADC wouldn't work with the GSX audio out, as it's just analogue as it would be from anything else with analogue audio out. Another thing to rule out though.
Without a DAC to connect to the ADC though, there's no way of knowing if it's actually working. The problem could lie with that.