Google reports the problem is resolved: https://www.google.com/appsstatus#hl=en&v=issue&sid=1&iid=a8b67908fadee664c68c240ff9f529ab

We are continuing to monitor for any further issues.

We are continuing to monitor this issue. Please continue to review the status at:
https://downdetector.com/status/gmail/
https://www.google.com/appsstatus#hl=en&v=status

There are currently known issues with Gmail. The problem has been acknowledged and they are working resolve the problem. Please monitor the their status page at https://www.google.com/appsstatus#hl=en&v=status. Alternatively please monitor the status at https://downdetector.com/status/gmail/ .
Next update at 6PM.