If you want to use the graphical user interface, you need the Gimp Tool Kit (GTK+) library to be installed. GTK+ is distributed under the terms of the GNU General Public License, and can be freely obtained from
http://www.gtk.org/ .Supported architectures include Linux as well as most Unix systems. These days, all popular Linux distributions come with GTK+ included. However, GTK+ is often split into a runtime package and a development package. To compile the graphical user interface, the development package is required.
On installation, GTK+ also installs a shell script gtk-config, that can be used to locate the installed library files. You should make sure that the directory where gtk-config is located is in your PATH environment variable.