I was having a similar issue, and @Coop had the solution for me on the thresholdx forum thread linked above.
Short version:
I recently purchased the M20 Collection through X-Aviation
To enable the Reality XP gauges, you need to hover your mouse on the left side of the screen.
A little tab will appear, that will let you bring up a menu, hit "settings" and then you can switch to the GTN from Reality XP.