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One possibility would be to load your Gmail account in Mail in IMAP. Would work with no action required from you and will not mess you Gmail "web" account, but the inconvenience is that you would have to have Mail open in order to have tasks be moved automatically to OmniFocus.

(in any case, loading Gmail in IMAP (and *not* the default pop3) in Mail is generally a good idea: spotlight indexation, integration with iPhoto, AddressBook, etc.... all this without messing with your original Gmail mails (while POP3 *will* not have your email synced))