From Google AppStatus page:

1/24/14 2:12 PM | We’re investigating reports of an issue with Gmail. We will provide more information shortly.

1/24/14 3:23 PM
The problem with Gmail 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.