Thank you for your post. There is currently no explicit setting that allows you to specify which GPS to use within Avenza Maps. If the external GPS is compatible with your device and is connected correctly, Avenza Maps will generally use this by default over the internal GPS of the device. We have tested with various Garmin and Bad Elf GPS receivers and find these to work the best with the app, so you should be able to use your Garmin GLO with no issues.
If you have any other questions or problems, please don’t hesitate to let us know, or contact our Support team directly through help.avenzamaps.com
Avenza Systems Inc.