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.