From glancing at the documentation for both of those services, neither appear to be OAuth 2.0, or at least have any mention of supporting the client (aka JavaScript) flow for OAuth 2.0 authentication. If that is indeed the case, then the gwt-oauth2 library won't be able to help you.
The library is designed to support any OAuth 2.0 provider that supports the pure JavaScript flow, which means it doesn't require a server. If you're using OAuth 1.0 or a service that doesn't support the JS flow, you'll have to have a server involved to handle the auth flow. I suggest the
http://code.google.com/p/google-oauth-java-client project if you decide you need to go that route.
- Jason