Mail searches an online database to see how to connect to an account; it defaults to IMAP for GMail because that can work better than POP, but you can override it later.
Try This:
Tell Mail.app not to take the account on line immediately, then edit the account to switch it from IMAP to POP.
(Personally, I'd switch GMail to IMAP instead.)
By default Mail will synchronize all messages locally; you can verify that in the account settings. My main worry is that POP3 likes to delete what it's downloaded, and if you don't let it do so then it has an annoying tendency to get confused and endlessly re-download messages it already has.
(POP3's a rather unintelligent protocol, and Gmail's a rather noncompliant mail server.)