There isn’t currently a way to specify the GPS receiver that is used within Avenza Maps on an Android or iOS device. As long as you have an external GPS unit that is compatible with the application, like a Garmin or Bad Elf GPS receiver, and it is configured to the vendor’s standards, it will be utilized in the app. The best way to be sure of this is to do some testing in the app with the device while connected and disconnected to the external GPS.
All the best,
Avenza Systems Inc.