Google reported a problem with Gmail today and not long after said it was resolved:

3:02 AM: We’re investigating reports of an issue with Google Mail. We will provide more information shortly.

3:43 AM: The problem with Google Mail should be resolved. We apologize for the inconvenience and thank you for your patience and continued support. Please rest assured that system reliability is a top priority at Google, and we are making continuous improvements to make our systems better.
Users were experiencing 502 errors and latency when accessing email.