Any particular reason you want to use RPC to Django instead of a web service from GWT application? I think using JSON communication between Django and GWT is much easier than RPC.
The main challenge in using RPC is that your Django and GWT client should communicate via a common protocol. On Django side you need a module that translates the GWT RPC calls to something Django understands and then convert the Django results into something GWT client application understands. In a nutshell RPC calls are serialized text stream sent over http via post method. On the server side you should be able to translate that into the right data structure by deserializing it. So you need a library that will do this serialization and deserialization for Django based on GWT-RPC protocol.
Useful links:
About GWT RPC:
Google documentation : RPC
GWT RPC in a nutshell
GWT + Django:
How to use Django with GWT?
Using GWT with Django using xml
Google Web Toolkit like application in Django
Pyjs
You can also look at http://pyjs.org/ which is a python to JS compiler and use it with Django. The great thing about pyjs is that you are still in Python world :)
http://www.derekschaefer.net/2011/02/08/pyjamas-django-pure-win/
http://gdwarner.blogspot.no/2008/10/brief-pyjamas-django-tutorial.html