Google’s App Status Dashboard notes many of their apps, including Gmail, Drive, Classroom and Calendar, had service disruptions this morning, but as of now are all again working as expected.
1/29/19 — Google’s Apps Status Dashboard notes:

“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, 7:50 AM detailing when we expect to resolve the problem. Please note that this resolution time is an estimate and may change.

Users are getting 404s when signing-in