Using the RequestFactory to pass oauth token with gwt-oauth2

I have successfully obtained a token using the gwt-oauth2 api in my GWT project.  Using the RequestFactory for RPC, how do I relay that token to a backend of my choosing? For example, if I were using app engine, how does app engine now know who the requesting user is?   What must be done from the client-side GWT code to pass that token?

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