Ground Overlay with Transparency

I have an online map that contains a ground-overlay image, and would like to make the image semi-transparent so that the base map will show through. Is it possible to add a transparency value to this overlay?

http://www.tpwd.state.tx.us/fishboat/fish/recreational/lakes/ingulf1c.phtml

Here's the code for the ground overlay:

var imageBounds = new google.maps.LatLngBounds(
  new google.maps.LatLng(25.71438633861514, -98.33555959121725),
  new google.maps.LatLng(30.40813339247205, -93.57953893270167));


function initialize() {
  var map = new google.maps.Map(document.getElementById("map"), {
    center: new google.maps.LatLng(28.0041,-96.3618),
    zoom: 7,
    mapTypeId: 'roadmap'
  });
  overlay = new google.maps.GroundOverlay(
  '/fishboat/fish/recreational/lakes/images/statemaps/gulfregion.gif',
  imageBounds);
  overlay.setMap(map);