It was recommended in the IRC room that I send this email.
I struggled all yesterday failing to get GRUB (and thus GuixSD) to install in ways that I was positive had worked before.
Turns out the installation USB had booted UEFI rather than BIOS, so it refused to install what I understood to be the default (BIOS GRUB), even though I had done and partitioned everything correctly, and there were no meaningful hints as to why.
It would be nice if there was a little note about this in the install guide, something along the lines of, `EFI/BIOS detection for GRUB depends on how the installation image was booted.' (or whatever it actually does, I'm not sure really).