Hi,

thank you for the report!  This is definitely a problem.

It seems that the Linux kernel already started up so this is not a bootloader
problem but rather a problem with the Linux kernel.  There have been lots and
LOTS of amdgpudrmfb changes in the Linux kernel lately - but our GuixSD
release is pretty old.

There should be a new release soon - the 0.14.0 release is from Dec 2017 and
the Linux kernel from back then had some problem modesetting AMD cards.

See also https://apple.stackexchange.com/questions/211260/grub-2-error-fb-switching-to-radeondrmfb-from-efi-vga

As a workaround,

(1) Wait until the grub menu shows up
(2) Press e
(3) Go to the end of the line which says "linux"
(4) Add a space
(5) Add nomodeset
(6) Press F10