Hello Jonathan,

> I'll try to bisect the issue as I have no an "easy" reproducer :)

I experienced the exact same issue. To be able to boot again, I booted
from a Guix System USB drive, hitted 'c' in Grub, then typed the
following commands:

--8<---------------cut here---------------start------------->8---
set root=(hd0,gpt2)
configfile /boot/grub/grub.cfg
--8<---------------cut here---------------end--------------->8---

Now regarding the cause of the issue, it appears that running the
following command from a Guix "master" Grub fixes the issue:

--8<---------------cut here---------------start------------->8---
sudo /gnu/store/w8v5d1i6xfqlpj78w89jg1x7f8dchh4k-grub-efi-2.06/sbin/grub-install --boot-directory /boot --efi-directory=/boot/efi
--8<---------------cut here---------------end--------------->8---

So this means that we probably have an issue related to the Grub package
on core-updates-frozen itself. Both master and core-updates-frozen are
using Grub 2.06 release.

The issue might then be related to the GCC 10 switch?

Here are the log files of Grub installation of master and
core-updates-frozen branches. I edited the /gnu/store/xxx-grub-efi-2.06
strings to remove the noise.

It looks like the core-updates-frozen Grub has a new .note.grub.property
section but I cannot find any other noticeable differences.

To be continued,

Mathieu