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.
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