Doesn't the following work?
osm.wiki/Google_Maps_Example
you could also have a look at the page source of
http://87.106.29.46/expopark/?page_id=71#karte

Alternatively, you could also use OpenLayers,
osm.wiki/OpenLayers
which can also show Google maps, cf.
http://openlayers.org/dev/examples/google.html