I get that occasionally in my pop accounts, too. If I wait a few minutes, it comes back and downloads the entire message. Who know why. Now I have IMAP for exchange and it seems to be working fine. So, I have to agree, even though I don't have gmail.
I am using Gmail IMAP and couple of mins back I got this thing which said "this message has not been downloaded from server". I just went back to the main mail screen and returned to the Gmail account and the message got downloaded. Just try to go back to the previous screen and re-enter to go into the Gmail account. It should work.