I know there's a
GWT Plugin for Struts that allows interaction. It doesn't look like it's been worked on in a while, but it is not
impossible for Struts and GWT to play nice.
Struts is an implementation of the
Front Controller Pattern for use when developing traditional web sites/applications (by "traditional" I mean that they adhere to the HTTP request/response cycle. Struts 2 supports AJAX through plugins to access other API's like Dojo and JQuery).
On the other hand, GWT is an API to simplify the development of rich web applications that use AJAX instead of reloading the page with each button press (as is typical with HTTP request/response). One interesting feature of GWT is that you write the application in
Java, then the GWT compiler generates an HTML/JavaScript/AJAX front end for the browser.