Question I have connected the S/PDIF out of my G-System to my audio interface. The signal in the input shows -6dB. Why is that

This corresponds to the Boost headroom specified in the Glb Levels menu. The output level of the G-System is decreased with X dBs (6 dB by default) to allow this level to gain some dBs when the BOOST switch is pressed. If you want to get the highest possible level in the digital output while recording, press the BOOST switch, or set the boost max to 0 in the Glb Levels menu.