Looks legit to me.
Key is in the https://www.google.com/, and the padlock in the BRight corner.
Have you done any unusual activities recently? Such as send more mail in a week than your average?
It happened a few months ago with my paypal account. Been dormant for over a year, then I made a single US$25 payment to somewhere and I got this same message (from paypal, of course) I had to reauthenticate twice to release the payment.