use this approach:
var lat=new_marker.getPosition().lat();
var lng=new_marker.getPosition().lng();
var target = new GLatLng(lat,lng);
var center= new GLatLng(yourLat, yourLng);//your gps position
var distance = center.distanceFrom(target) / 1000;//meters to km
if(distance < radius){//where radius = 5Km
new_marker.setMap(map);//add to map
}