There’s currently no CUPS service for GuixSD—i.e., nothing you could directly add to the ‘services’ field of your ‘operating-system’ configuration.
If you would like to contribute one such service, that’d be welcome! The relevant documentation starts at: http://www.gnu.org/software/guix/manual/html_node/Defining-Services.html The CUPS daemon is needed for printers directly connected to your machine via USB, for instance. Printers accessible over the network are typically accessible automagically via the GTK+ printer dialog, thanks to DNS-SD (Avahi). Ludo’.
