According to debian/changelog, Debian began using the custom username
Debian-gdm for the 'gdm' system user in gdm 2.30.5-7 in 2011

  * gdm3.init: chown the configuration to Debian-gdm, to avoid umask
    issues. Closes: #603510.

This means that tools to configure GDM (like the GDM Settings app
which was briefly in Debian) probably don't work in Debian. Guides to
configure GDM probably won't work. See https://bugs.debian.org/1107934

We probably need to add a postinst script to try to move user
configuration over. Technically, any system that had a dconf profile
to modify gdm already had their modifications no longer applied when
updating from bookworm to trixie.

I am not currently working on this bug, but I'm filing this as a
wishlist bug and to provide some background.

Thank you,
Jeremy Bícha

The big problem with having 'gdm' as a system user is that if your name
is something like George David Miller, it's completely reasonable that
you would want to use your initials, gdm, as your own login name. (And
perhaps our users have already done so.)

Arguably the GNOME display manager's system user account should be _gdm
(which would be the preferred name if gdm was being newly added to
Debian, as per Policy §9.2) but that's no better than Debian-gdm in
terms of whether upstream guides are going to work.

It's relatively normal for usernames and username policy to vary between
distros - see also _flatpak (flatpak in many distros) and messagebus
(dbus in many distros) - so anything that interacts with a different
package's system user, for example gdm-setup, should expect to need a
build-time option for the name of that user.

     smcv