Being that the AP is controlled by the G3X via the CAN bus I don't think your plan is feasible.
In talking with the folks at Trutrak a couple years ago they agreed with the approach of switching GPS sources. If the G3X dies the most simple and effective method is to have an independant switch.
If the G3X dies you can always leave the CAN bus leads tied to the Gemini and just rotate to serial inputs from the GTN.
Cheers.
Garmin's CAN bus is not compatible with TruTrak or a std serial bus.
If you're talking about using a TT AP with Garmin then yes you can switch 232 sources to the TT.
Not sure why anyone would use a TT AP with a G3X though.