Is there a way you can use KDE filepicker with GNOME apps? I guess if I somehow trick the app into thinking it is running under KDE it will then use KDE filepicker. Am I right?
The GNOME filepicker is part of the GTK toolkit and stays the same no matter what desktop environment you use.
There is no way to use another filepicker other than programming your own and patching GTK to use it.