In Ubuntu 12.10 I run KDE once (usually I use GNOME), after that all windows, menus, icons and so on looks like KDE, not GNOME theme. How enable again GNOME look?
I had the same problem (Ubuntu 13.10) when I tried kubuntu-desktop today, and I found two annoying packages in my terminal with some regex → gtk2-engines-oxygen gtk3-engines-oxygen. So I tried to remove them with the following command and it works.
Hope I helped you.