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 requires at least version 2.2 of the GTK library.

You can download it from http://gtk.org/.

Precompiled libraries for windows can be found at http://gimp.org/win32/, among other places.