Testing (unit or integration) GWT applications has several difficulties, recently i have meet a Java testing framework for GWT applications (gwt-test-utils), this library provides utilities for both unit test and integration test.
I like "GWT + Spring web application testing" feature of this library. Unfortunately the spring-sample of this project in github was not complete, I forked it in my github account. You can get it from this link and see how to integration test GWT + Spring web applications using gwt-test-utils.
Have a nice time.
