UPDATE: The problem with Gmail should be resolved for the vast majority of affected users. Google will continue to work towards restoring service for the remaining affected users, but no further updates will be provided on the G Suite Status Dashboard. https://www.google.com/appsstatus#hl=en&v=issue&sid=1&iid=e3aaf7adc786e8e60c44f238c29223c9

Google is investigating reports of an issue with Gmail. The affected users are unable to access Gmail. This is affecting users around the world.

Users may experiences issues signing into all Google services. 

We will post a followup when they have resolved the issue(s). 

You may also browse to the Google’s App status page for this and other related news.