Google is aware of the problem and is working on a fix. The issues seem to only be affecting those using Gmail with an IMAP mail client, and results in error messages and other problems. These issues shouldn't affect those using the Gmail app or Gmail on the web.
We're aware of a problem with Gmail affecting a significant subset of users. The affected users are able to access Gmail, but are seeing error messages and/or other unexpected behavior.
Google plans to offer an update on the situation soon. The company first started investigating reports of a disruption this morning at around 10:20 a.m. EDT, and expects to issue an update on the situation by 12:15 p.m. EDT, though they note that this time could change.