The .fit file for my edge500 always has the GPS info in addition to the data from other ant+ sensors. Garmin Connect always seems to use the sensor data even when it's wrong compared to the GPS data. It would be nice to be able to tell GC to ignore data from a bad ant+ sensor or tell it to only use the GPS data.
While riding the other day my gsc10 started putting out bad data and caused my ride to show way to much mileage. There were other errors in the reporting on the unit and in Garmin Connect due to the use of the GSC10. However the gps data logged in the .fit file was correct.
Thankfully Strava either only uses the GPS data, or it detected the ant+ data as bad (compared to the gps data) So after uploading to Strava, which only used the GPS data, I then exported the .gpx and upload that to Garmin Connect to get a more accurate account of my ride.
I'm reasonably sure the bad data from the GSC10 was due to a low battery. But I have not confirmed that.