You may want to checkout this project https://gerrit.googlesource.com/gwtjsonrpc/
It's a library created in order to support a code review system for Android, Gerrit, but it's a stand-alone module meant to be embedded into any GWT project, not just Gerrit.
A reasonable tutorial is probably the README in the top level of the directory. It's quite similar to standard GWT RPC but it uses JSON encoding. It also has built-in XSRF protection.
