GWT is just a way of avoiding JavaScript, and that is its main purpose, to write front-end code in Java. Honestly, I just don't see the point if your server is not application (i.e., the one running in your server) it's written in Java. As a concept is very interesting and it could offer some extra performance against poorly written JavaScript.
But if you truly want to learn JavaScript (and you should want), avoid this tool because you are going to see any JavaScript, since the generated JavaScript is not even legible. Anyway, in the future it is much more likely that you are going to work with JavaScript than GWT, so you should know at least the basis of JavaScript.
My advice is starting from the very beginning to learn the language itself, then the DOM and other basic things like AJAX. You can take a book to learn them, but I think the most solid option is directly the Mozilla Developer Center (MDN). It has really good documentation, and the best place to start is from this guide.
Then, you may consider doing real things ;) For that, a JavaScript framework is practically required. I recommend you jQuery because it is very popular, quick and sexy. My choice could be also MooTools, because you can actually write code in a reusable and sane way (unlike jQuery) and it is very Object Oriented, but I don't know if it is really the best option to someone that is starting.