The problem was first identified on Monday, though Google has apparently taken the service completely down today for further repairs. Google has said that fixes are ongoing, and that the service should be restored shortly. Some reports have indicated that only certain aspects of Gmail were affected. People relying on the IMAP functionality of the service reported that while it still worked, it was unstable. People logging onto the web client were deprived of access, and are still having issues at of the time of writing.
This is another in a series of Gmail outages, including one this year and two others in 2008. It was because of those issues that Google crafted their service status page, the first step in immediate feedback on whether or not a Google service is functioning.