Google has identified an issue with Gmail, and is investigating the issue. We will keep this page updated, and report here when the issue is resolved.
"We're investigating reports of an issue with Gmail. We will provide more information shortly. The affected users are able to access Gmail, but may not have access to the most recent data.
Users can still access emails on Inbox by Gmail and Gmail mobile."
See here for more information from Google: https://www.google.com/appsstatus#hl=en&v=issue&sid=1&iid=9e0952d1c05f27c23a14755d9c438a53