How do I garbage collect a Guix System generation?

Running sudo guix gc --list-roots on my Guix System I see a lot of system generations lying around, going all the way back to when I first installed the system.

$ sudo guix gc --list-roots
...
/var/guix/profiles/system-22-link
/var/guix/profiles/system-1-link
/var/guix/profiles/system-10-link
/var/guix/profiles/system-11-link
/var/guix/profiles/system-12-link
/var/guix/profiles/system-13-link
/var/guix/profiles/system-14-link
/var/guix/profiles/system-15-link
...

These are not garbage collected when I run sudo guix gc --delete-generations=30d as I would expect. Why is this and how do I GC the system roots?