If it is about the backup of a big Gmail account, then it should be a tool which supports incremental backups and filters by date, because otherwise it will have to download the entire Gmail account for every backup update, what becomes quite problematic when you are working with thousands of emails.
The Kernel Email Converter tool allows to perform the job. This software connects to Gmail, exports to PST keeping labels, attachments and folders structure, supports incremental export (after the initial backup it downloads only the new emails, not all the account again), and works with several accounts at once if needed.
However, in my opinion, manual synchronization through IMAP via Outlook cannot be applied in case of such big mailbox. It works for small accounts, but is slow and has risks to lose some emails during the process.
