It is not GWT. It is very similar to the other JS-heavy Google Apps (GMail, Docs, Buzz, etc.) I'm not sure how many details of those are public, so I'll leave it at that.

Edit: Actually, apparently parts of the stack have been open-sourced:

http://code.google.com/closure/

http://code.google.com/p/protobuf/

http://code.google.com/p/google-guice/