You're conflating a desktop environment and a graphics toolkit. KDE <-> GTK+ ~= apples <-> oranges.
Futhermore, no, GTK's HiDPI support does not in fact force a scaling factor for the whole desktop. You can turn it on app-by-app. The fact that whatever desktop you're using doesn't utilize this capability isn't GTK+'s fault, it's the desktop environment's fault.
Now having said this, you could be entirely correct that for the time being KDE offers a better HiDPI experience than a GTK+-based desktop, but again, that has really nothing to do with the toolkits themselves.
Futhermore, no, GTK's HiDPI support does not in fact force a scaling factor for the whole desktop. You can turn it on app-by-app. The fact that whatever desktop you're using doesn't utilize this capability isn't GTK+'s fault, it's the desktop environment's fault.
Now having said this, you could be entirely correct that for the time being KDE offers a better HiDPI experience than a GTK+-based desktop, but again, that has really nothing to do with the toolkits themselves.