None of the class names used are GWT class names, so I doubt it. It'll be the output of an internal template engine. Interestingly, AFAIK, most of Google is not written in GWT -- anyone know any production Google stuff that is? (I think Wave is.)
Wave is indeed developed in GWT, and the Adwords management interface was rewritten in GWT in the recent years (as mason55 mentioned). They talked a lot about it during Google I/O 2009 (the session videos are still available online: http://code.google.com/intl/fr/events/io/2009/sessions.html#... ).
What's interesting, is that with the Adwords UI, Google started using GWT on an enterprise app, and thought about ways of writing such an app. This project lead them to introduce many best practices for large GWT enterprise apps:
- MVP (Model View Presenter) and testable UIs: MVP is good for SoC, but it's also very important if you want to unit test your UI logic in pure java (which is a lot faster than GWTTestCases)
Google I/O 2010's sessions also have a lot of talks about these subjects, and while the best way of writing GWT apps is not yet known, we are getting there.
3. It does have some weird IFRAME structures that are unlike what GWT uses.
4. It doesn't have any of the long hexadecimal identifier files that GWT uses to compile to different targets.
5. On the wire, it doesn't look at all like GWT AJAX requests.
6. Isaac Truett <itru...@gmail.com> says it isn't at http://groups.google.com/group/Google-Web-Toolkit/browse_thr... This was in 2007, but I don't believe they threw away a working product to do a rewrite in GWT, which was still not fully mature and is still hard to get just right. And if they did, they'd also have had to make it look like it is not written in that framework in every way I can think to check easily.
(EDIT) 7. Since nothing significant is written in GWT, they'd be trumpeting it from the rooftops if GMail was.
