GNOME desktop
The GNOME project provides two things: The GNOME desktop environment, an intuitive and attractive desktop for users, and the GNOME development platform, an extensive framework for building applications that integrate into the rest of the desktop.

GTK+ - The GIMP Toolkit
GTK+ is a multi-platform toolkit for creating graphical user interfaces. Offering a complete set of widgets, GTK+ is suitable for projects ranging from small one-off projects to complete application suites.

Gtk#
Gtk# is a .Net language binding for the gtk+ toolkit and assorted GNOME libraries.

GAPI
The GAPI tools are distributed with Gtk#. The tools make it easy create and maintain managed bindings to native libraries which utilize glib and GObject.