You could use something like lieer[1] which will also sync your notmuch tags with gmail labels.
If you don't want to sync the labels, I'm sure there are plenty of solutions out there to download Gmail to your local machine in a well known format like Maildir.
I strongly urge people to try notmuch. I switched to it in around 2011 and have never gone back.
You don't have to use Emacs as the client. There are plenty of other frontends (including aerc).
https://notmuchmail.org/frontends/
alot[1] is probably the most well known among them:
https://github.com/pazz/alot