The following example shows a simple Marker (standard icon) on Google maps for the location and when clicking the icon, it opens the info window. Can I show the info window
You can run "open" directly on page load should open the window automatically, without a click.
infowindow.open(map, marker);
However, this might not autopan the map properly in some cases, leaving the top of the overlay off the edge of the map.
A way that worked for me is to wrap it in a listener to the "tilesloaded" event on the map
google.maps.event.addListenerOnce(map, 'tilesloaded', function() {
infowindow.open(map, marker);
});