I'm having a similar problem but with an ArcGISDynamicMapServiceLayer.  If the google maps layer is the base layer, I have the same problem Lizd is having.  If I use the ESRI World-Imagery layer as the base layer and hide it behind the google maps layer, it only screws up if the AGDMSLayer is shown before a zoom.  After a zoom (whether the AGDMSLayer was visible or not), it has no problem.  It seems like it is using the wrong extent to query and display the map.  Is this a problem with the gmaps.GoogleMapsLayer?  Is this even supported by ESRI?  Any insights would be helpful.