GTK, GTK+, and Qt are GUI toolkits. These are libraries that developers use to design graphical interfaces, all running on top of the X Server or Wayland. These are things that you need to install as dependencies. They're the Linux "equivalent" to Windows' GDI/GDI+. When an application uses any of these, it will usually have a general "look and feel".
GNOME and KDE are Desktop Environments. GNOME primarily uses the GTK+ toolkit, while KDE primarily uses the Qt toolkit. There are applications designed for GNOME or KDE, such as a settings menu or a default music player, usually in the appropriate toolkit. These Desktop Environments have a set of utilities/window managers/design specification to create a more unified desktop. You can mix the two if you feel like it, but you may run into issues with colliding standards and applications (which is a bit more common on systems like Arch).
Unity uses many of the GNOME utilities and backends (Nautilus, Rhythmbox, gvfs, etc.), so Unity is more GNOME than KDE.
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https://askubuntu.com/questions/249150/what-is-kde-gtk-gtk-qt-and-or-gnome