If the Garmin app cooperates and shares data with any of the major data corps (Microsoft, apple, google, facebook group, etc), then whatever data Garmin has access to (including yours GPS) will find its way into those company's hands.
It depends on Garmin's privacy policy and app.
If the garmin app uses internet, then it could potentially be sending data elsewhere. If its fully offline and never gains internet access, then its fine. But if you have to connect it online every once in a while, then each time it can upload the data it has been gathering. This is only "IF" it gathers and sends data and "IF" it cooperates with those companies. Whether it does or not however is beyond my knowledge. Someone else has to speak to that.