ENABLE [ALL MAIL] IN GMAIL TO ARCHIVE

If you are archiving a gmail mailbox to your local hard disk, then you will need to enable [All Mail] in your settings so that you can access those old emails from your Outlook or Thunderbird client via IMAP. Once you a have enabled it, re-launch your email client, find the All Mail mailbox, wait for an hour or so and then start moving the old mails off to your local PST or whatever.