Getting a script to start after gdm3 starts but BEFORE login

Question

I'm trying to get a script to run after gdm3 starts but before a user logs in. Reason is the script needs an active monitor, but if there's a power outage, I want the system to boot back to functional when power is restored.

I've tried various iterations based on other answers, but none of them seem to be performing the way I need it to. I'm also trying to contain this in my custom systemd service definition and not touch any other service definitions, if possible. Here's my current (not working) iteration:

[Unit]
Description=Testing
After=network.target network-online.target gdm3.service

[Service]
Type=oneshot
User=root
Group=root

GuessMainPID=no
ExecStart=/bin/bash /home/user/test.sh
Restart=never

[Install]
WantedBy=multi-user.target

Reboot -> Nothing. After I login -> Nothing. I can manually start it and it is enabled. I reloaded the daemon after any edits and still nothing.

Answer 1

The logs were confusing because it looked like the script was triggering but nothing actually happened. I figured it out based on the logs and quite a bit of Googling.

Looks like gdm has 2 X sessions; 1 pre-login and another post-login. When I set my DISPLAY in the lines of the script along with Xauthority, I had the DISPLAY correct, but the Xauthority was the one that was post-login. I just needed to SSH in before loging in, do ps a | grep X to find the proper Xauthority (which is the only one at this point since I haven't logged in through gdm), change the script with the correct Xauthority, reboot, and seems to be good now.