The Speed/Cadence Bike Sensor (GSC 10) over rides the data from the GPS satellites.
Garmin GPS fitness devices record speed and distance using GPS satellites, except when using the GSC 10. The accessory uses a magnet on the wheel and the bike profile's wheel size to calculate distance and speed with improved accuracy.
The GSC 10 is useful when traveling through tunnels, heavily treed areas and on rides with substantial changes in elevation where GPS can be limited. When the device is recording distance with GPS, it is recording the distance from one point to the next without factoring in the elevation ascent or descent. The GSC 10 gives a more accurate reading by recording distance based on wheel rotation.
If preferred to use GPS data for speed and distance, the magnet on the wheel will need to be removed.