Information You Need to Know

From Google:

Backing up your mail with POP - Gmail Help:

Here's how to download a copy of every message in Gmail to an email client:

Sign in to Gmail.

Click Settings at the top of any Gmail page, and open the Forwarding and POP/IMAP tab.

Select Enable POP for all mail (even mail that's already been downloaded).

Click Save Changes.

Open the mail client you've configured for Gmail, and check for new messages.

Gmail messages are downloaded in batches, so it may take time for everything to appear in your mail client.