I guess it switched desktop environments on you? If you’re logged out there’s supposed to be a way you can switch between desktop environments. It makes sense that the GNOME Settings app would only change wallpaper settings for GNOME, which I think is the main Ubuntu desktop environment. Are you sure you didn’t upgrade to a version of Ubuntu that uses XFCE instead of GNOME?
I guess it switched desktop environments on you? If you’re logged out there’s supposed to be a way you can switch between desktop environments. It makes sense that the GNOME Settings app would only change wallpaper settings for GNOME, which I think is the main Ubuntu desktop environment. Are you sure you didn’t upgrade to a version of Ubuntu that uses XFCE instead of GNOME?