Google Web toolkit and AppEngine are working together very well. But I found a lot of tutorials showing GWT script and AppEngine are sitting in a single box. But I believe that should be able to send RPC calls to a separated servers. Google Map GWT library is to send RPC calls to Google Map Server. So, there should be a way to write an application with a separated GWT client script and AppEngine.

(Note: I found after the service class casted to ServiceDefTarget, I can use setServiceEntryPoint to a remote server. But our web browsers has Single Origin Policy, which blocked the request to remoted servers. I am checking whether any work around.)