The article below really describes the Takeout method, but in many cases that is really not easily usable elsewhere. Thunderbird installs on Windows, macOS and Linux, and will connect to your GMail and fully sync that mail offline for you. This allows you not only to have a fully searchable offline repository of your mail, but it will also connect to any other mail service, allowing you, if you so wish, to use other mail providers whilst retaining all the history of your older e-mails.
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