There are different ways to integrate with other frameworks and tools such as those you mentioned. The easiest way is through a plugin. Grails plugins are easy to install and use and there are currently plugins for integrating with Wicket, GWT, Struts 1, Flex and most of the popular Ajax frameworks. Check the Plugin Portal for even more: http://grails.org/plugin/home
More fine grained integration can also be done, but would best be discussed on a case by case basis.