Gmail and Google Apps have noticed on Saturday that the Google Internet Authority G2 has become untrusted due to an expired digital certificate in the chain of trust. The company informed users that it is aware of the problem, but it hasnt provided any details on the problem. Google immediately started the operation to fix the issue, at 3:46PM the issue has been resolved. The problem with Gmail should be resolved. We apologize for the inconvenience and thank you for your patience and continued support.”]
Source: http://securityaffairs.co/wordpress/35689/digital-id/google-internet-authority-issue.html