In verifying the installation image, I type this line in the terminal

$ gpg --verify guixsd-usb-install-0.12.0.x86_64-linux.xz.sig

Please note that I borrowed this line from the instructions for the binary
installation, but I am using it for the GuixSD verification.

What i get is:

gpg: non ci sono dati firmati
gpg: can't hash datafile: errore durante l'apertura del file

"non ci sono dati firmati" means "there are no signed data"
"errore durante l' apertura del file" means "error during opening of the
file"

This is on Fedora 25 with an Italian locale

Of course, this is the ls of the folder I run this line into

$ ls
Fedora-Workstation-25-1.3-x86_64-CHECKSUM
Fedora-Workstation-Live-x86_64-25-1.3.iso
guixsd-usb-install-0.12.0.x86_64-linux
guixsd-usb-install-0.12.0.x86_64-linux.xz.sig

Thanks for your patience