Sorry about my tardiness to this thread.
You can put a switch on both the RS232 and ARINC inputs to the autopilot. If you have the RS232 set to the G3X, then you want to make sure the ARINC is coming from the G3X as well. However, if you want to have the autopilot to follow steering commands directly from your navigator, you can switch the ARINC to the GPS.
I hope this clears it up some. Basically the G3X interface requires both serial and ARINC.