This file is indexed.

/etc/gdm3/PreSession/Default is in gdm3 3.18.3-0ubuntu2.

This file is owned by root:root, with mode 0o755.

The actual contents of the file can be viewed below.

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#!/bin/sh
#
# Note that any setup should come before the sessreg command as
# that must be 'exec'ed for the pid to be correct (sessreg uses the parent
# pid)
#
# Note that output goes into the .xsession-errors file for easy debugging
#
PATH="/usr/bin/X11:/usr/X11R6/bin:/opt/X11R6/bin:$PATH"

if [ ! -d /run/systemd/system ]; then
  /sbin/initctl -q emit desktop-session-start DISPLAY_MANAGER=gdm
fi