get distance based on latitude and longitude

GMaps3 offers under the geometry library google.maps.geometry.spherical which contains:

  

Useful functions to calculate angles, distances and geodetic areas. The default radius is the radius of the Earth of 6378137 meters.

To include it you can do so mediate:

<script type="text/javascript" src="https://maps.googleapis.com/maps/api/js?v=3&libraries=geometry"></script>

Then you can use google.maps.geometry.spherical.computeDistanceBetween which:

  

Returns the distance, in meters, between two LatLngs. Optionally, you can specify a custom radio. The default radius is the radius of the Earth.

Like this:

var distance = google.maps.geometry.spherical.computeDistanceBetween(myLatlng, myLatlng2);
// distancees un valor en metros
console.log((distance/ 1000) + 'Kms');

Example:

var map;
function initialize() {
  var myLatlng = new google.maps.LatLng(41.38, 2.18);
  var myLatlng2 = new google.maps.LatLng(41.35, 2.18);

  var myOptions = {
    zoom: 13,
    center: myLatlng,
    mapTypeId: google.maps.MapTypeId.ROADMAP
  }
  map = new google.maps.Map(document.getElementById("map-canvas"), myOptions);

  var marker = new google.maps.Marker({
    draggable: true,
    position: myLatlng,
    map: map,
    title: "Your location"
  });
  var marker2 = new google.maps.Marker({
    draggable: true,
    position: myLatlng2,
    map: map,
    title: "Your location"
  });

  google.maps.event.addListener(map, 'click', function(event) {
    alert(event.latLng);
  });

  // AQUI calculo de distancia entre los puntos
  var distance = google.maps.geometry.spherical.computeDistanceBetween(myLatlng, myLatlng2);
  // distancees un valor en metros
  console.log((distance/ 1000) + 'Kms');
}

google.maps.event.addDomListener(window, 'load', initialize);

html,
body,
#map-canvas {
  height: 100%;
  width: 100%;
  margin: 0px;
  padding: 0px
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.0.2/jquery.min.js"></script>
<script src="https://maps.googleapis.com/maps/api/js?v=3&libraries=geometry&key=AIzaSyCK3ulYYM8GCwK-_HlZVNKvkgp-zvEebUM&"></script>
<div id="map-canvas" style="border: 2px solid #3872ac;"></div>

To obtain the distance between two points with latitude and longitude use the formula Haversine translated into javascript is as follows.

function getDistanciaMetros(lat1,lon1,lat2,lon2)
{
  rad = function(x) {return x*Math.PI/180;}
  var R = 6378.137; //Radio de la tierra en km 
  var dLat = rad( lat2 - lat1 );
  var dLong = rad( lon2 - lon1 );
  var a = Math.sin(dLat/2) * Math.sin(dLat/2) + Math.cos(rad(lat1)) * 
  Math.cos(rad(lat2)) * Math.sin(dLong/2) * Math.sin(dLong/2);
  var c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1-a));

  //aquí obtienes la distancia en metros por la conversion 1Km =1000m
  var d = R * c * 1000; 
  return d ; 
}