In Mission Planner, I set the GPS_NAVFILTER property to 8 (Airborne 4G) which seems to be the default anyway. I sent it up under a weather balloon to 34,000m and the GPS worked until 21,999m. The Ublox M8N receiver was on the outside of the payload so, despite the suspicious number, it probably froze. It was about -52C at that altitude.
The net payload will have two GPS units, a Here+ and another M8N, both set to Airborne 4G, both insulated to hold them at about 30C.
Not sure if that helps but it's all I did.