GDM3 Login List -- where does this list come from?

I manage a batch of diskless workstations and have *automatic* login
(AutomaticLogin options in /etc/gdm3/custom.conf) set up (for a guest
account). *Sometimes* people need to login with a specific personal account.
What I would like would be for the *automatic* login to have a delay. On
machines *with* disks, this can be done with the TimedLogin options in
/etc/gdm3/custom.conf. The problem is this: the logins are authenticated via
LDAP. The timed login seems to fail (hangs) *the first time* but works if I
*manually* login to the guest account first, since it is not listing the user
to be logged in (there is no "history" logins) -- on the machines with disks,
the history of logins is listed someplace. With the diskless machines, /var,
/etc. and /root are ramdisks, created and populated at boot time (from
templates loaded from the server).

With lightdm under Ubuntu 14.04 there were files under /var/cache that
contained the the login names that showed up in the user list. What is the
equivalent under GDM3/Ubuntu 18.04?