Hate to be the bearer of bad news, but no there is not.
What you could do, is do a SSO but against Google Apps.
Most of the documentation about SSO asumes that you need to authenticate against LDAP/Ad. But I have developed a few that authenticate directly against Google.
There are 2 reasons: 1 - clients hate the "non-corporate" login screen 2 - I can enforce password expiration (part off the SSO is developing the password change)
In theory, you could develop this in App Engine.
Good luck!