When mounting a bluetooth device behind the panel, don't forget that the bottom of the panel is open. I believe this is true for all models except the -12. The opening generally allows for good bluetooth reception when the GTX is mounted behind the panel
On my RV-6a, I have a GDL 39 mounted aft of the baggage compartment and the baggage bulkhead blocks the bluetooth signal. The GDL and GTX bluetooth signal strength may not be identical, but I would not expect you to get good bluetooth reception with a device in the tailcone. I cannot pair my GDL to a phone or tablet reliably. It is hardwired to my display, so bluetooth was not a requirement for me.
The primary communication from the G3X to the GTX is multiple hard-wired RS-232 connections. This is how the GTX is controlled and how ADS-B Traffic and Weather are displayed on the G3X displays. The bluetooth is only used for displaying the traffic, weather, and attitude data on a compatible Garmin portable device or tablet.
Thanks,
Levi Self
Team X Project Manager