Gmail on iPad not working ..error message
Try a couple things:


Delete the problem gmail account from your iPad, and then set it up again.  See if email works.
If #1 doesn't work, then it could be possible you have too many devices accessing that email account at the same time (ie., iMac/notebooks/etc with the Mail app left open).  Try closing the mail app on your other devices and see if that works.  I believe gmail has a limit of 10 devices that can access the same email account at a given time.  A mail app left open counts as one.
Lastly, make sure you've inputted the correct password.  You might want to change your password in gmail.com and then re-enter it in your iPad.


Hope this helps!