At the moment, there's not a way to set default resolution or monitor settings for GNOME.

I see two justifications for this:

1) An administrator should be able to set the default monitor setup for new users.
For example, a special multi-monitor setup, or a monitor/graphics card that detects the wrong resolution (I have a television that does this), or possibly defaulting to a lower resolution than the detected (maybe a weak graphics card on a native hi-resolution screen?).

2) Use this "default" monitor.xml/settings for GDM. This could fix issues where the login display is on the wrong monitor, or possibly a few reasons as stated in point number 1. Basically, this would be a way to override the default GDM settings. /var/lib/gdm/.config/monitors.xml currently does this but doesn't seem to work for Wayland, see bug 748098.

Rui Matos suggested a "system wide option", but I think this a bad name to use as I don't think it should overwrite any existing monitor.xml configurations of other users. I think it would make more sense for it to act as the resolution of GDM and default for new users. Hence calling it something like "Apply as default".