A word of warning, there is plenty of google brain cancer in GWT, one of my personal favorites that elicited "wtf..."s frequently in a previous occupation:
To this day I still can't fathom why someone thought it'd be acceptable to violate conventions by providing is<Type>() methods that do not return boolean results.
In my 1 year of using GWT, I've never had to use that class. Also, you're never required to use GWT's implementation of JSON, you're free to write your own.
It lets you write both server & client code in java, and then the client code is compiled to highly optimized javascript.