The simplest way would be to use GWT's built-in JSON API. Here's the documentation. And here is a great tutorial on how to use it.
It's as simple as this:
String json = //json string
JSONValue value = JSONParser.parse(json);
The JSONValue API is pretty cool. It lets you chain validations as you extract values from the JSON object so that exceptions will be thrown if anything's amiss with the format.