Really? I don't mind how GNOME apps look on KDE. I agree it doesn't look or feel "native" but I find them to be just as usable as they would be if they were used in GNOME. But in any case Qt is intended as a cross-platform toolkit. GTK is not really intended to be that, it is its own thing.
Edit: Actually part of the problem might be your theme. In XFCE/KDE/etc you probably want to disable the theming for GNOME apps and just use Adwaita, in my experience GTK themes have never really worked well with GNOME apps for various technical reasons. However I think in the coming years libadwaita will improve the situation.
Edit: Actually part of the problem might be your theme. In XFCE/KDE/etc you probably want to disable the theming for GNOME apps and just use Adwaita, in my experience GTK themes have never really worked well with GNOME apps for various technical reasons. However I think in the coming years libadwaita will improve the situation.