This incident has been resolved.

Google is reporting an issue with the Google Sign Sign On feature. Guru users that are not logged into Guru and attempt to log in via Google SSO might not be able to. More information can be found on Google's status page at: https://www.google.com/appsstatus#hl=en&v=issue&sid=1&iid=e3aaf7adc786e8e60c44f238c29223c9