

Any halfway decent desktop email client will do the job—people have already listed several. I use claws-mail, but getting it to work with GMail involves the computer equivalent of doing a triple backflip through a hoop, so you may want to go with something more common.
It looks like the simplest method is to use rEFInd + shim or PreLoader—see The rEFInd Boot Manager: Managing Secure Boot.
If you need a last-ditch option or want to get your hands dirty, the Gentoo wiki has recipes for enrolling new EFI keys by hand. This is a rather tedious procedure, but should work for any distro that comes with efitools, sbsigntools (for signing the kernel and modules, if not already signed), and openssl (for key generation).