We are not sure exactly what cable you would need for the GoPro camera, but we can point you to the specifications for the video input on the GDU. These can be found on page 26-23 of the G3X Touch Installation Manual
You would need a composite (Analog) video signal from your camera to input the G3X Video In. I don't know what the output of a GoPro is (Analog or Digital), you'd have to look at you user manual. You'll have to change the end of your video cable to a BNC for the G3X