Comment 1 for bug 1727466

Dirty hack
Make sure numlockx is installed and
sudo sed -i -e 's/#WaylandEnable=false/WaylandEnable=false/g' /etc/gdm3/custom.conf

as sudo:
create a script named num.sh in /usr/local/sbin/num.sh

sudo pico /usr/local/sbin/num.sh
and put this in it:

#!/bin/bash
gdm=`pidof gdm3`
until [ $gdm -gt 0 ] do
su - gdm -s /bin/sh
export $(dbus-launch)
DISPLAY=:0
export DISPLAY
export XAUTHORITY=/run/user/121/gdm/Xauthority
numlockx on
done
exit

make it excutable:
sudo chmod +x /usr/local/sbin/num.sh

create a /etc/rc.local file
sudo pico /etc/rc.local
put this in it:

#!/bin/sh -e
#
# rc.local
#
# This script is executed at the end of each multiuser runlevel.
# Make sure that the script will "exit 0" on success or any other
# value on error.
#
# In order to enable or disable this script just change the execution
# bits.
#
# By default this script does nothing.
/usr/local/sbin/num.sh &
exit 0

make it also executable:
sudo chmod +x /etc/rc.local

Should be working numlockx on gdm screen.

Place your script also in /etc/gdm3/PostSession/Default
sed -i '$ i\/usr/local/sbin/num.sh &' /etc/gdm3/PostSession/Default
without it: when you log out,,, no numlockx,,