You can always setup your GMail account on your Mac as a POP3 account, and then flag GMail to mark all messages new. The next mail check would cause your mail app to download all the messages on teh GMail server to your mail client. Once they are in your mail client, you manage them locally in there, and it won't matter if the GMail host account goes away or not, because POP3 works with the local data once it is downloaded form the server.
Be advised that doing this can cause you to redownload thousands (or hundreds of thousands) of emails depending upon your email volume and how long you have been using GMail. If I do this, my GMail wants to re-send close to 6GB of data to me again because I have been using GMail for about as long as it has existed, and I average 100-200 emails a day on that account.