Take a look at the MVP architecture -
http://code.google.com/webtoolkit/articles/mvp-architecture.html
Even though you don't actually need an MVP paradigm to create a 'Multi-page' gwt application, you will be better off following it in creating your apps.
Basic idea is, you create different view-presenters that will act like different 'pages', and the app controller activates the correct one based on the logic you provide.
EDIT - you can even create multiple html-pages with their own onModuleLoad(), You should prefer that only if - there are logically 2 different applications or you are modifying an existing webpage and cant do without it. I believe you don't have those issues based on your question. Let me know if its otherwise.