At the moment Garmin Connect for Android is not able to read .gpx with mutliple tracks. It just imports the first track and ignores the others. Therefor this tool creates per track a new gpx file <trackname>.gpx. Those file can then be used by Garmin Connect for Android.
Provide the path to the .gpx file as an argument. See the example below.
Run the example go run main.go ./example/etappen.gpx. This should create a folder example/etappen and write all tracks in separate files.