You might want to create a WTP project using the GPE first, because to use the modules you need to create an EAR project. This is described here:
https://developers.google.com/appengine/docs/java/webtoolsplatform
You will create modules, each of which will be a dynamic web project. Your GWT application will be one of them. And follow the instructions for the modules here:
https://developers.google.com/appengine/docs/java/modules/
Please note that when you right-click on your project there will be two elements related to Google: Google and Google App Engine WTP. Commands related to the application engine in the Google menu do not seem to work. Instead, use the commands associated with the application engine in the Google App Engine WTP menu.
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