More information can be found at http://www.gtk.org/.
Common Lisp GTK bindings, etc. on Cliki:
- cl-gtk2 - CL-GTK2 is a Common Lisp GUI library that provides access to the Gtk+ cross-platform GUI library in idiomatic Lisp style
- GNOME - GNOME is the GNU Object Model Environment, intended to provide a user-friendly "desktop environment" for UNIX operating systems
- GTK binding - This page lists Common Lisp bindings for the GTK GUI toolkit
- gtk-cffi - GTK-CFFI is a Common Lisp GUI library, that provides concise and idiomatic syntax to use GTK+ library (for now GTK+ 3.0)
GTK can also be used with Lisp through GTK-server.