GTK (and its helper libraries: glib, gdk, pango, atk, etc.) is a graphical toolkit library, and is the basis for the most popular Freeciv frontend. Most GNU/Linux distributions come with GTK, but you will probably have to install the development libraries to be able to compile freeciv. Freeciv 2.0 required at least version 2.2 of the GTK library. Freeciv 2.6 was the last version officially supporting Gtk2 clients in addition to Gtk3.
You can download it from gtk.org, for an overview see wikipedia:GTK+. Precompiled libraries for windows can be found at gimp.org/win32, among other places. The preferred way to obtain GTK+ in binary form for Windows NT 6 is to use MSYS2. The last version supporting Windows 2000 was 2.16.6 (2010).