As you can see here GateIn 3.3 has integration with SAML2. From GateIn 3.4, we added possibility to integrate with 3rd party SAML vendors Salesforce and Google Apps. At this moment, you can already use latest version of GateIn SSO component and follow this wiki page where you can see the details about integration.
If you want to see GateIn and SAML in action, you can look at my screencasts. In first screencast, you can look at general SSO scenarios for GateIn and SAML2 integration, which are also described in GateIn reference guide.