gtk-fortran is a three-stage rocket:

• I use the src/cfwrapper/cfwrapper.py script to parse the GTK .h header files and automatically generate the src/*-auto.f90 files containing interfaces to the functions of the GTK libraries. See the How gtk-fortran is generated page for more details.
• The src/gtk.f90 and the src/*-auto.f90 files constitute the sources of the gtk-fortran interface. You can use all their functions to create your graphical user interface in your own program.
• But you can also use the gtk_hl high level interface for gtk-fortran (src/hl-*.f90 files) which offers a simplified interface to the most useful GTK functions.