This tutorial by Matt Raible is easily the best one I've seen so far on OAuth and gwt. He also has a very good picture depicting the authentication flow, which I always find help. However, as Matt himself says, the solution is not 100% reliable, but it might still get you part of the way.
With this in mind, it might be better to just go with a pure javascript implementation of it. You'll find one such implementation right here. This SO thread might come in handy to you if you chose that path.
Best of luck to you.
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