Just look at the examples and test pages which come with the War Release.
To use the TagTemplateServlet you have to add some lines to the web.xml:
<!-- Servlet for Groovy Template Language -->
<servlet>
<servlet-name>gtl</servlet-name>
<servlet-class>gvtags.TagTemplateServlet</servlet-class>
</servlet>
<!-- Standard Action Servlet Mapping -->
<servlet-mapping>
<servlet-name>gtl</servlet-name>
<url-pattern>*.gtl</url-pattern>
</servlet-mapping>