FWIW, I used gmvault a long time ago (N.B. I typically deactivated individual .eml.gz compression in favor of a more global compression with a squashfs archive of the gmvault backup). Since I found not very practical to search through that archive, I developed https://github.com/karteum/gmvaultdb in order to convert it to a local sqlite DB (I made it for personal use and chose to keep the schema simple to have simple queries for my case. If the DB was to grow bigger I'd probably use a different schema e.g. to place the contacts/from/to/cc in separate tables... I also chose to extract attachments directly on the filesystem rather than put them in the DB - which enable direct access and deduplication)

(N.B. it can also process the mbox produced by https://takeout.google.com/ with the caveat that in some case Takeout permanently losses some information because of a bug with older encodings so I'd always prefer a backup using gmvault or imap)