Google has reported a problem affecting access to Gmail for a significant subset of users:
https://www.google.com/appsstatus#hl=en&v=issue&sid=1&iid=d8c52d15f9cab60823bbd15d46a00708 We're aware of a problem with Gmail affecting a significant subset of users. The affected users are unable to access Gmail. We will provide an update by 1/29/19, 12:50 PM detailing when we expect to resolve the problem. Please note that this resolution time is an estimate and may change.