That is correct and it isn't something we've thought about before as gmail isn't a general purpose cloud storage system.
If you could get your gmail backing up to your drive then you could certainly access it with rclone but I couldn't see an easy way of doing it.
Using google takeout looks like the best way of backing up gmail
I wonder if that can be automated...
Using https://takeout.google.com you can schedule a backup every 2 months for a year to google drive which would be a start, but not a great backup.
I think takeout is very hard to automate according to what I've been reading.