Did this used to work or have you just set it up for Gmail access? Did you do it in accordance with: http://support.apple.com/kb/ht4810 ?
Are you accessing it via cellular network or WiFi? If WiFi then is this your usual network?
If it has been working fine for some time and now suddenly you get this message having changed nothing then it is most likely a temporary glitch at Google end or your ISP perhaps.