Once the system had finished booting, and going to the login screen, the login screen was not a resolution my (graphics card + monitor) supported, and the screen would look 'diagonal'.
I would have to, manually, do ctrl+alt+'-' to lower the resolution by one setting, for the gdm3 login to then look correct.
My video card: voodoo3 3000
My monitor: ACER X183h
After researching via google, on and off for weeks now, I now have an nice one-line change to a gdm3 configuration file that works for me.
1. sudo vi /etc/gdm3/Init/Display
2. Find the location in the script where it has
3. Append directly under those two entries an xrandr command, using the resolution you want gdm3 to default to.
For me, this is what I added:
# added xrandr command to set the default resolution to 1280x720
xrandr --output default --mode 1280x720
Now my gdm3 login is the resolution I want :)