Google is aware of a problem with Gmail affecting a majority of users. The affected users are able to access Gmail, but are seeing error messages and/or other unexpected behavior. Google will provide an update by April 4, 2015 12:00:00 PM PDT detailing when they expect to resolve the problem. Please note that this resolution time is an estimate and may change.
smtp.gmail.com is displaying an invalid certificate.
You may use the gmail.com web interface as a workaround.
NB. This is not affecting any of our clients as we don't use SMTP; however, we are posting this as Google has made us aware of the issue. As a result, we have set our Google Apps Gmail status to operational.