Comment 21 for bug 460328

I may disagree. If "empty" = "no override", wouldn't it be logical to leave it empty and use system-level settings, until user decides to set something explicitly for himself?
In case of gdm, which is a system tool, not a user-owned profile, it is even more appropriate to follow system-level settings, but with possibility of optional gdm-level override.

another issue is /desktop/gnome/peripherals/keyboard/general/defaultGroup key.
Its default "-1" value (no defined default layout) is rather useless, it brings uncertainty in managing layouts when switching between several existing and new windows. "0" would be more appropriate.
But that isn't as important as the first issue.