Yes, you are free to use GWT with any server-side templating tool.
GWT’s network operations are all asynchronous, or non-blocking. That is, they return immediately as soon as called, and require the user to use a callback method to handle the results when they are eventually returned from the server. Though in some cases asynchronous operators are less convenient to use than synchronous operators, GWT does not provide synchronous operators.
Here’s the GWT development cycle:Use your favorite Java IDE to write and debug an application in the Java language, using as many (or as few) GWT libraries as you find useful.
Confirm that your application works in each browser that you want to support, which usually takes no additional work.