Change the resolution of the login screen

I am trying to follow this approach but with no luck.
The file should have been copied.

-rw-r--r-- 1 gdm gdm 542 Sep 25 13:30 /var/lib/gdm

Any ideas?

Thanks!

Thank you for the response!
I actually figured it out. It is now using the gdm3 directory so this worked:

sudo cp ~/.config/monitors.xml /var/lib/gdm3/.config/monitors.xml
sudo chown gdm:gdm /var/lib/gdm3/.config/monitors.xml