I believe we are not using the --builtin session for GNOME Shell but it looks like at least the GDM greeter is still relying on that, as evidenced by the logs:

gnome-session-binary[962]: GnomeDesktop-DEBUG(+): Not systemd managed, will not move PID 1193 into transient scope

services.xserver = {
  enable = true;

  displayManager.gdm = {
    enable = true;
    debug = true;
  };
  desktopManager.gnome = {
    enable = true;
    debug = true;
  };
};

cc @NixOS/gnome

Related:

• non-systemd support might be removed from gnome-sesion – although --builtin currently remains
• Pantheon’s Gala is still using --builtin: pantheon: manage user session with systemd #140429
• So is flashback cc @chpatrick