Adding Gmail to iCloud as a backup

I run Gmail in Apple Mail on my computer. I think it would be a good idea to add my Gmails to iCloud for safekeeping. To achieve that, I probably need to add Gmail to my dormant iCloud Mail account.

I can't find how to do it.

But I also wonder, if I ever need the backed-up emails, how to get them back to my computer, or will they just sync?

Thanks for your help!