Got confirmation on how to connect up an ammeter shunt so my battery charging status can be displayed on the G3X.

From SteinAir:
You can add a shunt on the output side (the side connecting to the system, not the battery terminal) of the master relay and then connect two wires from the small terminals on the shunt to pins 36 and 37 on the GEA24 J243. Then go into the config mode and in the Engine and Airframe section, change shunt 1 to read Battery Amps. This will now display amps on the G3X.

This is the recommended shunt, which I just ordered.