I recently installed Fedora 22 with KDE Plasma 5 and I noticed that the GTK+ applications appearance settings are not available in System settings, but kcm-gtk module is installed.
true, we don't yet have a good gtk-configuration option, currently only have:
* kde4-based kcm-gtk + xsettings-kde
* kde-config-gtk: included in plasma-5.x releases, but does not integrate with xsettings-kde
kde-gtk-config is in fedora now, even though it's still not our preferred way of doing it (ie, it doesn't integrate with xsettings-kde like kcm-gtk did).