The Gtk-fortran project is a set of Fortran modules that allow the gtk+ libraries to be called from Fortran. It can be used with both recent versions of Gtk+-2 and also with Gtk+-3.
In addition to the standard Gtk+ routines (along with Glib, Gdk and Cairo) there is a high-level interface which uses Fortran’s optional argument system to create a programming interface to the most common widgets that should be more comfortable to IDL (Interactive Data Language) programmers.