GPS External Signal can receive location from your USB connected Garmin GPS and use that location as the main system one in your Android device. Thanks to that any other app that uses either GPS or Network location will receive the one from your Garmin GPS instead of the internal GPS.
In case that your Android device can't get GPS position after using GPS External Signal app try resetting either the GPS or Network provider from the top right app menu.
If logging to a file is enabled than GPS External Signal logs every location it receives from GPS to a GPX file as a track.
The logs are stored on your internal storage in folder GPS External Signal. Every time logging is started a new .gpx file is created with the name GPS_Log_yyyy-MM-dd_HH-mm-ss.gpx
If you'd like the ability to change log filename or log folder, let me know
Both these apps require you to grant them USB permission for your GPS.
You need at least one OTG cable connected to your Android device.
For example if you have micro-USB port on your Android device then you can connect to it with a micro-USB to USB OTG cable. After that you can use any ordinary USB cable to connect the OTG cable with your GPS.
You can use USB Host Diagnostics app to test if USB Host is supported on you Android device.
If there is everywhere Yes or Full under Kernel and Verdict then USB Host is supported.
In order to receive the external GPS position in other apps you need to enable Allow mock locations under Developer options.
To enable Developer options go to
Settings > About > Build number and tap it 7 times
Best way to test if your Android device will be able to connect to your Garmin GPS is through GPS Protocols app.
After you click Connect the app will try to connect to the GPS and load it's name and supported protocols. If everything goes fine you'll see similar info dialog as in the image above.
You are golden! Both your Android device and your GPS are fully supported by GPS Exernal Transfer.
Nothing's lost yet. Please email me the whole list of your protocols and contents of the app log, which you can access through the top right menu button. I'll look into it and hopefully I should be able to add support for your GPS.
Unfortunately GPS External Signal won't work with your GPS and your Android device.