Section 1.7 of the G5 installation manual provides guidance for connecting a wide variety of navigators from portable GPS units to panel mount IFR navigators using RS-232 and in some cases CAN bus. A G3X system is not required.
For a portable navigator like the Aera 660, the G5 receives both lateral and vertical VNAV guidance over RS-232 using a NMEA format.
We are not aware of any method to use an iPad as a navigator.
As of November 2016, there is NO WAY to get NAV data from an iPad to any other device. Yes you can look at the iPad and program another device to try and do what you see on the iPad but it will not run an autopilot as there is NO serial (RS232) port to output the data needed.
If someone knows a way, a new thread needs started to teach the world how to do it.
Yes, the G5 installation manual shows how to use one of the two serial ports on the 796 (configured for NMEA Out) to provide lateral and vertical (VNAV) guidance to one, or multiple, G5 units.