Create a new POP3 account in Outlook. As soon as it is created, disconnect the internet.
Click File -> Options -> Advanced. Click Send/Receive button.
Select Edit, then your account, then account properties. In the advanced tab check "Leave a copy on the server".
Now, re-connect the internet and send / receive. It could take a long time depending on how many emails you have.
You will now be downloading a copy from Gmail without syncing (meaning, if you were to delete an email in Outlook, it still exists on the server).
The negative is, no syncing, so if you send an email via Outlook, it will only appear in the sent folder in Outlook, and not in Gmail.
Any way, if this has solved the problem, go back to accounts and remove the original IMAP account. If not, then delete this new pop3 account.