In order to activate mirrors automatically on boot one has to add geom_mirror_load="YES" to loader.conf(5). It is not documented in gmirror.8, however.
Reported by: Michiel van Baak <firstname.lastname@example.org>
The problem is actually bigger than anticipated. All the other files in the head/lib/geom/ subdirectories for the other GEOM modules also do not have this information in the SYNOPSIS section. I've opened a review for gmirror.8 here: