After some assistance from Garmin support, I was ready to configure the GMA 350c for voice commands. The GMA comes with software disabled discrete inputs. Two of these are used for PTC (Pilot and Copilot). With config. software from Garmin and a USB/Audio cable I was able to enable the two discrete in question. Immediately the voice commands worked and I ended up spending an hour giving commands to the GTN and GMA. Very much like the wife do to me...
Only issue was that the read back was somewhat choppy, like someone turned on and off the intercom. Need to look into the noise/interference.