If you have more than one GPS source in use at one time, the ForeFlight app will prioritize which GPS source to use based upon a hierarchy. Here is the GPS priority list starting with the highest priority source.
-Stratus, Sentry, Scout
-FreeFlight, Stratux, Lynx
EXAMPLE: If you have a GPS equipped iPad, Garmin, and a Stratus all in use at the same time, the GPS used will be from the Garmin followed by the Stratus and then by the iPad's GPS receiver.