If your eyes go to tears everytime you turn on GIMP or Firefox from the clash of QT/gtk themes, you'll be very happy to know that someone has gone through alot of trouble to make KDE Breeze themes for GTK 2/3.
You can download their lovingly crafted themes here:
You can install it by copying the extracted theme folders to : /usr/share/themes/
Go to: System Settings > Application Style > Gnome Application Style
Choose Breeze-gtk or Breeze-dark-gtk under GTK themes and you're good to go.
If you have a custom colors (aka you've deviated from blue for toggles, scrollbars, etc), it may still clash a bit as the GTK themes use use Breeze's blue on white color scheme. I may at some point make a fork that changes all the blue assets to orange. If you're interested, let me know and I'll post it up here.