You can only change the standby frequency on the GTR 200 itself, as it is with most radios. You need to flip flop with active to tune the active. This is easily done from the HDX. The only thing you can't do operationally from the HDX is turn on monitoring of the standby channel. In practice I use the GTR200 flip flop switch because it's a single button press most of the time, but I never tune the radio with its own controls.
RV-9A #91081, C-GCPT
Dynon SkyView HDX, IO-320 and WW 200RV C/S. Flying as of August 6, 2018
Added GPS 175 and authorized for IFR April 1, 2021
Interactive map of all of my flights here