GWT, Spring@Grails (Java/Groovy), GAE - play together in harmony
There is the Wentity project which shows as a proof-of-concept how to incorporate these technologies and let them play together in harmony: It uses Google´s App Engine infrastructure for a Spring based version of Grails which also uses Google´s Web Toolkit for RIA content. The project is in a structure-finding draft phase. Nevertheless it can be built and deployed on GAE. Use it as a template for your own project to save time! Along with Groovy you can also use your Java code in this project. Also it has the capability to i18n enhance your web apps to offer internalized/translated versions of them. The Wentity demo site gives you an impression for the functionality which is already been implemented and working.