I hit the same error. I began with the error and the fix from https://issues.guix.gnu.org/23286 and then put xscreensaver into my screenlock section of my config:
--8<---------------cut here---------------start------------->8--- (list (service gnome-desktop-service-type) ;(screen-locker-service xscreensaver) ;; this initializes, but shouts a "do not run as root" error instead of running my images, and doesn't accept my password ;;; other stuff ) --8<---------------cut here---------------end--------------->8--- Now I have 'xscreensaver: don't login as root' on my screensaver, and no images. As mentioned earlier, only the xscreensaver-auth program should be setuid, not the whole thing, apparently.