README.md

groundOverlay.setOpacity()

Change the opacity of the ground overlay. The value range is from 0.0 to 1.0.

<div id="map_canvas">
  <span  class="smallPanel">
    Opacity: 0.0&nbsp;<input id="opacityRange" type="range" min="0" max="1" step="0.1" value="0.5">&nbsp;1.0
  </span>
</div>

var slider = document.getElementById('opacityRange');

var bounds = [
  {"lat": 40.712216, "lng": -74.22655},
  {"lat": 40.773941, "lng": -74.12544}
];
var div = document.getElementById("map_canvas");
var map = plugin.google.maps.Map.getMap(div, {
  camera: {
    target: bounds,
    padding: 40
  }
});
map.one(plugin.google.maps.event.MAP_READY, function() {

  // All gestures (such as pinch-zooming) are disabled.
  map.setAllGesturesEnabled(false);

  // Add ground overlay
  map.addGroundOverlay({
    'url': "../images/newark_nj_1922.jpg",
    'bounds': bounds,
    'opacity': 0.5
  }, function(groundOverlay) {

    // If the slider is moved, change the ground overlay opacity.
    slider.addEventListener('change', function() {
      groundOverlay.setOpacity(this.value);
    });

    // Set the current value as opacity.
    groundOverlay.setOpacity( parseFloat(rangeSlider.noUiSlider.get(), 10) );
  });
});