OK, I deleted my previous answer because it turned out to be exactly what you didn't want.
I don't know how well it works with GWT, but we use the json-lib library to serialize objects in a normal Java project where I work.
It can create a JSONObject directly from a JavaBean, then use the resulting JSONObject's toString() method to get the actual JSON string back.
Likewise, it can also turn JSON back into a JavaBean.
