I found this too when I first purchased the plane. I discovered that when clicking on the screen of the GPS, and the popup appears. The power button does nothing.
HOWEVER, clicking on the power button on the GPS from the 3D cockpit does turn it on!
But although that turns it on, the switch button for COM1 doesn't work properly (I haven't raised this as an issue yet though), it seems to set both active and standby to 121.5 eventually. (Doesn't actually swap back and forth).
I have a Saitek radio panel which does seem to swap correctly though, so this is what I use.
@virtualgaa How do you turn the Avionics Master Switch on/off exactly if the aircraft doesn't have one? I have a problem with the GTN 750 where it always is off. Obviously, some people are using it OK. Used to work OK for me, but months ago it suddenly stopped working. Is there an approved fix or workaround?
So our customers can discuss flight sim in general and our products. We do keep an eye on the forums but if you need any sort of technical help the simplest solution is to contact us by raising a ticket. However, it's often the case that other forum users can answer questions - especially ones that are related to flying rather than simply software.
As to your specific question, it's not my bag but someone else may have an answer. Otherwise, our support team will pass your ticket over to the XP devs and see if there is an answer.
@MonsieurBon Thanks for the suggestion, but I finally figured out what needed to be done in my situation. I had my UI scale (x-Plane Graphics Settings) set to 150%, which apparently causes the grey arrow to appear way up at the top of my monitor. And when I click the grey arrow, only the bottom row of the panel selector appears, which prevents me from selecting any of the buttons on the top 2 rows. When I changed my UI scale back to 100%, that solved my problem completely. The grey arrow now appears about 1/3 of the way down from the top of my monitor, and when I click it, the entire panel selector appears with all 3 rows accessible. All good now, thanks again for your help!