year/month/day/some-email-id.eml. That .eml format is just plain text containing message headers and plain text email content in the original encoding (no automatic conversion to �!). Also it has created an sqlite database for correspondence between GMail labels and messages. It's possible to incrementally update the backup, so it doesn't take long to update the backup.
ripgrepor other text searching utilities. Or you can import it all in Mozzila Thunderbird (using their Import/Export add-on available from
Options -> Extensions) and search from there. However, note that labels will be lost in this way (since they are in the sqlite database). I don't care much about labels to be fair.
offlineimapwhich looked good, but it took forever to sync my email (after a week it was still working on it!) and it was often failing with intermittent errors, so I ditched it. Also it required setting up a non-trivial config for GMail before you can even start using it.