This file is indexed.

/etc/dbus-1/system.d/gdm.conf is in gdm 3.0.4-0ubuntu15.

This file is owned by root:root, with mode 0o644.

The actual contents of the file can be viewed below.

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<!DOCTYPE busconfig PUBLIC
 "-//freedesktop//DTD D-BUS Bus Configuration 1.0//EN"
 "http://www.freedesktop.org/standards/dbus/1.0/busconfig.dtd">
<busconfig>

  <!-- Only root can own the service -->
  <policy user="root">
    <allow own="org.gnome.DisplayManager"/>

    <allow send_destination="org.gnome.DisplayManager"
           send_interface="org.gnome.DisplayManager.Manager"/>
    <allow send_destination="org.gnome.DisplayManager"
           send_interface="org.gnome.DisplayManager.Display"/>
    <allow send_destination="org.gnome.DisplayManager"
           send_interface="org.gnome.DisplayManager.LocalDisplayFactory"/>
    <allow send_destination="org.gnome.DisplayManager"
           send_interface="org.gnome.DisplayManager.Settings"/>
    <allow send_destination="org.gnome.DisplayManager"
           send_interface="org.gnome.DisplayManager.Slave"/>
    <allow send_destination="org.gnome.DisplayManager"
           send_interface="org.gnome.DisplayManager.Session"/>
    <allow send_destination="org.gnome.DisplayManager"
           send_interface="org.freedesktop.DBus.Properties" />
    <allow send_destination="org.gnome.DisplayManager"
           send_interface="org.freedesktop.DBus.Introspectable"/>

  </policy>

  <policy context="default">
    <deny send_destination="org.gnome.DisplayManager"
          send_interface="org.gnome.DisplayManager.Manager"/>
    <deny send_destination="org.gnome.DisplayManager"
          send_interface="org.gnome.DisplayManager.Display"/>
    <deny send_destination="org.gnome.DisplayManager"
          send_interface="org.gnome.DisplayManager.LocalDisplayFactory"/>
    <deny send_destination="org.gnome.DisplayManager"
          send_interface="org.gnome.DisplayManager.Settings"/>
    <deny send_destination="org.gnome.DisplayManager"
          send_interface="org.gnome.DisplayManager.Slave"/>
    <deny send_destination="org.gnome.DisplayManager"
          send_interface="org.gnome.DisplayManager.Session"/>
    <deny send_destination="org.gnome.DisplayManager"
          send_interface="org.freedesktop.DBus.Properties" />
    <allow send_destination="org.gnome.DisplayManager"
           send_interface="org.freedesktop.DBus.Introspectable"/>

    <allow send_destination="org.gnome.DisplayManager"
           send_interface="org.gnome.DisplayManager.Display"
           send_member="GetId"/>
    <allow send_destination="org.gnome.DisplayManager"
           send_interface="org.gnome.DisplayManager.Display"
           send_member="GetRemoteHostname"/>
    <allow send_destination="org.gnome.DisplayManager"
           send_interface="org.gnome.DisplayManager.Display"
           send_member="GetSeatId"/>
    <allow send_destination="org.gnome.DisplayManager"
           send_interface="org.gnome.DisplayManager.Display"
           send_member="GetX11DisplayName"/>
    <allow send_destination="org.gnome.DisplayManager"
           send_interface="org.gnome.DisplayManager.Display"
           send_member="GetX11DisplayNumber"/>
    <allow send_destination="org.gnome.DisplayManager"
           send_interface="org.gnome.DisplayManager.Display"
           send_member="IsLocal"/>

    <allow send_destination="org.gnome.DisplayManager"
           send_interface="org.gnome.DisplayManager.LocalDisplayFactory"
           send_member="CreateTransientDisplay"/>
    <allow send_destination="org.gnome.DisplayManager"
           send_interface="org.gnome.DisplayManager.LocalDisplayFactory"
           send_member="SwitchToUser"/>
    <allow send_destination="org.gnome.DisplayManager"
           send_interface="org.gnome.DisplayManager.LocalDisplayFactory"
           send_member="StartGuestSession"/>

    <allow send_destination="org.gnome.DisplayManager"
           send_interface="org.gnome.DisplayManager.Manager"
           send_member="GetDisplays"/>

  </policy>

  <policy user="gdm">
    <allow send_destination="org.gnome.DisplayManager"
           send_interface="org.gnome.DisplayManager.Manager"/>
    <allow send_destination="org.gnome.DisplayManager"
           send_interface="org.gnome.DisplayManager.Display"/>
    <allow send_destination="org.gnome.DisplayManager"
           send_interface="org.gnome.DisplayManager.LocalDisplayFactory"/>
    <allow send_destination="org.gnome.DisplayManager"
           send_interface="org.gnome.DisplayManager.Settings"/>
    <allow send_destination="org.gnome.DisplayManager"
           send_interface="org.freedesktop.DBus.Properties" />
    <allow send_destination="org.gnome.DisplayManager"
           send_interface="org.freedesktop.DBus.Introspectable"/>
  </policy>

</busconfig>