In POP settings in gmail settings, you get an option where you can ask it for retrieval of all messages throug POP access even if it's already downloaded. Use this and then download messages in outlook.
Quote:
1. Status: POP is enabled for all mail that has arrived since Jun 10
Enable POP for all mail (even mail that's already been downloaded)
Enable POP for mail that arrives from now on
Disable POP
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You will see something like that in forwarding and POP/IMAP. Click on the radio buttone next to Enable POP for all mail (even mail that's already been downloaded) and save the settings.
If you want to backup your mail on hard disk, you can create a separate folder for the mails you want to save and backup the pst file for that. Don't remember the exact procedure.