If you use a native email client and this client saves every mail on disk, you have a chance to have it backed up with everything else. It's still far to be the generic scenario, I think, but it should raise a bit the number of people having their gmail backed up.
I use IMAP clients on all my computers to access gmail so they inherently have a local backup. Then at the beginning of each month I go to Google Takeout - https://www.google.com/settings/takeout - to download all my Google stuff and add it to a backup that ends up in Dropbox. (I should also add that I run my own domain and email so gmail is just a backup account.)
However on checking takeout it turns out the one thing they don't provide is gmail! 8 of the 15 items backed up are related to Google+ though.
