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In short, you write, test and debug your client-side code in Java and let GWT cross-compiler convert it all into cross-browser JavaScript. When compiled the Client-side is now pure JavaScript and HTML.
GWT Cross-Compiler compresses the JS naming and obsfuscates your code to the shortest possible length.
With GWIT you "Only pay for what you use!"
So, GWT Cross-Compiler prunes unused classes, methods and attributes, from your code.
In fact, GWT can generate faster JavaScript than you can write by hand!
In the ideal world, the Java developer would never have to write JavaScript code.
This is the fulfilled promise of GWT...
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With GWT, Google bets on the 6 milions Java developers community out of here.
Open Source and Community Initiative
GWT is not only a Google thing, since its an Open Source initiative under a friendly Apache 2.0 license.
The GWT community exists and well alive...with a developer forum of over 13 000 members, many third party development tools and libraries including an amazing visual editor (GWT Designer) and many books.
GWT is improving quickly. For example. , accessibility support is a main addition to the version 1.5 now in Release Candidate 2.