Yes, you are free to use GWT with any server-side templating tool.
With GWT development, your Java client code gets compiled into equivalent JavaScript that is loaded into your host pages. The generated product is totally independent from the server-side technology that you choose to use in your web application.
You can go ahead and use your favorite server-side templating tool and include your template directives into your host pages along with your GWT-generated JavaScript files. The server-side technology you're using to realize the templates is invisible to the browser and works just as it does without your GWT modules.
With GWT development, your Java client code gets compiled into equivalent JavaScript that is loaded into your host pages. The generated product is totally independent from the server-side technology that you choose to use in your web application.
You can go ahead and use your favorite server-side templating tool and include your template directives into your host pages along with your GWT-generated JavaScript files. The server-side technology you're using to realize the templates is invisible to the browser and works just as it does without your GWT modules.