Allegedly, on or about 01 June 2013, Anthony sent:
You *could*, I believe, uninstall GNOME and reinstall it.
I doubt that will reset how it behaves. User settings are stored in the user's homespace, packages don't create or remove those settings when installing or removing packages, users do while using them, and settings will be reapplied to a reinstalled Gnome.
Create a new user. That user will have the default settings. You could compare them.
You could switch over to using that new user, and copy your files across.