In short, "Yes". One of the benefits of Linux is it's ability to share common libraries/dependencies among different running applications. Because of this, many users are firmly rooted to GTK, QT, KDE , etc. and will refuse to install apps that have dependencies falling outside of their chosen camp. This line of thinking can have very practical benefits on a low resource machine. Imagine three applications, all based in different tool-kits are running on a single computer. Those applications will then need all their own tool-kit libraries loaded into ram in order to function, even though many of those libraries perform similar functions. Now imagine three apps all running on a some computer, but they are all Gnome-based. In this case, all three Gnome apps can share the same libraries in your ram. As you can see, tool-kit loyalty can certainly decrease your ram usage. As well as decrease data transfer from your hard disk as the system will be retrieving less dependencies.
The flip-side to this is that if you have a relatively modern computer you may not notice any difference at all. Multi-core processors, solid-state drives, and gobs of cheap ram make many of these considerations moot. Your biggest concern may actually be that a KDE app doesn't look "right" on your GTK+ desktop.
In the final analysis, you would have to gauge yourself as to whether your machine is bogging down under the load. If it still feels like normal, then don't worry about it. Enjoy using the applications you like. If it's dragging, then you may have to reconsider looking at some GTK options for future use. Good luck, hope this helps!