You can embed Spring SAML Extension (product built on top of OpenSAML) into your application. It will enable you to act as a SAML 2.0 Service Provider.
You can see a working demo of such approach at http://saml-federation.appspot.com/ which shows a Spring SAML sample application connected to SSO Circle IDP.
Shibboleth doesn't provide a SAML SP component which can be embedded into applications. The Shibboleth SPs act as plugins to Apache or IIS web servers and cannot be used with Google App Engine.
You can also see the Spring SAML as an example of SAML 2.0 WebSSO implementation to follow in case you'd like to write it youself. Latest documentation is available in Spring repos, sources are at GitHub.