at work I'm currently preparing a presentation about GWT. Just some Basic facts - Pros/Cons etc.
In my opinion one big advantage is that GWT is completely free. Its licensed under Apache License 2.0.... but why? Sure its good and Google gets good credit for developing it. But what benefit Google gets from it?
Google's business is the web, and it means that as much people using the web as much revenue they do. They promote technologies that open the web to everyone.
They open-sourced GWT in 2006, in the ages when everything run in the desktop. But they visioned that all of those could be run in the web, and they demonstrated that with GMail, Google Docs, and dozens of innovative ideas. That is the reason they did a product with the goal to enable productive development of high-performance web applications without the developer having to be an expert in browser quirks, XMLHttpRequest, and JavaScript
A best example of promoting the usage of the web, is Android. When all vendors were fighting in the mobile OS war, Google decided to enter in the game giving a free OS to anyone. That fact has contributed incredibly to the development of smartphones, as well as to allow thousand millions of people access the web. In sum more people can use google.
Another example is Chrome. They have achieved that browser vendors were more aligned with standards, and that the web development was easier. Polymer, Angular, Dart, Maps, and a very long list promote the same idea: easier access to the Web.
Obviously at the same time Google likes to research things that apparently does not have to do with the Web. But it's part of their strategy.
DISCLAIMER: This is just my opinion.