What do you mean by gtk under Qt? What about Tk apps, or Athena apps, or Motif, or GNUStep, etc.? There really is no such thing as a native Linux look and feel, you just happen to use a consistent selection of apps on your own system.
What I mean is that a Qt app will look native everywhere, and Gtk apps will look native in most places.
Most other toolkits, as you mentioned, don’t look native in many places, but Qt natively supports the CDE style, the Motif style, all different Windows and Mac styles, etc.
A Qt app looks native on Mac OSX, on Solaris, under Gnome, under KDE, under Windows XP, under Windows 10.
That’s why I avoid apps that aren’t either native or Qt anywhere, no matter if I am running Gnome or Windows, KDE or Mac.
Most GTK apps work nicely under Qt (although this is very suboptimal), but especially Qt apps look native everywhere. From windows to mac to KDE-environments to Gtk-environments.