IIRC, if you really want the HSI function, you need to also install the GMU 11?? Magnometer. The GMU 11 needs power, ground, and connects to the G5(s) via the CAN bus. If you want the G5 to fail-over to the "other" function, then they must be connected via the CAN bus...which you should wire up any way.
Each G5 has its own GPS Receiver and internal antenna. To get magnetic heading information you need the external GMU 11.