Le API di Google (https://developers.google.com/maps/documentation/javascript/overlays#PolygonOptions) permettono questo tipo di risultato utilizzando le linee e i poligoni (https://google-developers.appspot.com/maps/documentation/javascript/examples/polygon-simple).,
In poche parole dovresti aggiungere al javascript qualche istruzione simile a questa:
var flightPlanCoordinates = [
new google.maps.LatLng(37.772323, -122.214897),
new google.maps.LatLng(21.291982, -157.821856),
new google.maps.LatLng(-18.142599, 178.431),
new google.maps.LatLng(-27.46758, 153.027892)
];
var flightPath = new google.maps.Polyline({
path: flightPlanCoordinates,
strokeColor: "#FF0000",
strokeOpacity: 1.0,
strokeWeight: 2
});
flightPath.setMap(map);