Check if a GPS location is within a certain radius of another GPS location in android

It's very easy. When you add Marker into your Map just get distance from that Marker to your Current Location if distance is <= 20 then add that marker into Map. Like so

float[] results = new float[1];
Location.distanceBetween(oldPosition.latitude, oldPosition.longitude,
            newPosition.latitude, newPosition.longitude, results);