Count the distance between 2 set of GPS Coordinate (Latitude and Longitude).
Use CountDistance server action with :
Distance as result (Decimal)
IsMetricSystem as parameter to check using Kilometer (True) or Mile (False)
Lat_Center, Lng_Center as parameter the center coordinate
Lat_Compare, Lng_Compare as parameter of the location that will be compare to center
Can be used for Reactive app and Traditional web also.
Sorry for this, haven't successfully managed to changed the plugin type from mobile to service.