The latest guix system reconfigure (of yesterday) left me unable to login into
my X session.  guix system rollback DID NOT fix it.

I would enter my password and it would "try" to login and return right back to
the gdm login screen.

I've since removed gdm from my OS configuration (because I have to do actual
*work* on this computer), but I think it would have been enough to just
chown /var/lib/gdm and rm ~/.xsession-errors (!) in order to make it work
again.

Does that mean that user ids are non-reproducible?

Why not have user_id = hash(user_name) ?  Then they *are* reproducible.

(I've tried finding the spot where those user accounts are generated/updated
but so far have been unable to)

Anyway, this is just to record the problem and workaround.  I won't do
further research on this problem on it on this computer.

The "gdm" system account is gone by now because I've removed gdm from the
OS configuration--and I don't plan on adding it ever again.

For reference, in order to remove gdm from the system configuration in
/etc/config.scm, do:

(1) Replace %desktop-services by
 (remove (lambda (service) (eq? (service-kind service) gdm-service-type)) %desktop-services)

(2) Add (service slim-service-type) to SERVICES in /etc/config.scm

(3) guix system reconfigure /etc/config.scm