/lib/live/config/0080-gdm3 is in live-config 5.20170112+deb9u1.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 | #!/bin/sh
## live-config(7) - System Configuration Components
## Copyright (C) 2006-2015 Daniel Baumann <mail@daniel-baumann.ch>
##
## This program comes with ABSOLUTELY NO WARRANTY; for details see COPYING.
## This is free software, and you are welcome to redistribute it
## under certain conditions; see COPYING for details.
#set -e
Cmdline ()
{
	# Reading kernel command line
	for _PARAMETER in ${LIVE_CONFIG_CMDLINE}
	do
		case "${_PARAMETER}" in
			live-config.noautologin|noautologin)
				LIVE_CONFIG_NOAUTOLOGIN="true"
				;;
			live-config.nox11autologin|nox11autologin)
				LIVE_CONFIG_NOX11AUTOLOGIN="true"
				;;
			live-config.username=*|username=*)
				LIVE_USERNAME="${_PARAMETER#*username=}"
				;;
		esac
	done
}
Init ()
{
	# Disables both console and graphical autologin.
	case "${LIVE_CONFIG_NOAUTOLOGIN}" in
		true)
			exit 0
			;;
	esac
	# Disables graphical autologin, no matter what mechanism
	case "${LIVE_CONFIG_NOX11AUTOLOGIN}" in
		true)
			exit 0
			;;
	esac
	# Checking if package is installed or already configured
	if [ ! -e /var/lib/dpkg/info/gdm3.list ] || \
	   [ -e /var/lib/live/config/gdm3 ]
	then
		exit 0
	fi
	echo -n " gdm3"
}
Config ()
{
	# Checking if package is already configured
	if grep -qs AutomaticLoginEnable=true /etc/gdm3/daemon.conf
	then
		exit 0
	fi
	_GDM3_OPTIONS="AutomaticLoginEnable=true\n\
AutomaticLogin=${LIVE_USERNAME}\n\
TimedLoginEnable=true\n\
TimedLogin=${LIVE_USERNAME}\n\
TimedLoginDelay=5"
	sed -i -e "s|\[daemon\]|\[daemon\]\n${_GDM3_OPTIONS}|" /etc/gdm3/daemon.conf
	# Avoid xinit
	touch /var/lib/live/config/xinit
	# Creating state file
	touch /var/lib/live/config/gdm3
}
Cmdline
Init
Config
 |