I want to display a map with the infowindow box automatically displayed for the single marker on the page, much like http://code.google.com/apis/maps/documentation/javascript/ex
Your code is almost perfect. Except that instead of:
infowindow.open(Gmaps.map, marker);
You should have:
infowindow.open(Gmaps.map.map, marker);
Indeed, Gmaps.map is a container, Gmap.map.map is the google object.
I know these names are confusing. Sorry.
PS: be sure to put this code under the gmaps call in your view.