I checked out google takeout - but it only lets me backup the services I don't care about. Google for imapgrab. It's a tiny python program and it's as simple as can get.

Reading this link gave me the push I needed to finally backup my gmail. I've been worried about the [probably very low] risk of Google deciding to close my account, but I'm going to take it into my own hands now, and start using Gmail as my reader only.