If gtk-fortran does not fit your needs, there are many other libraries for plotting, drawing, making GUIs in Fortran:

• DISLIN by the Max Planck Institute for Solar System Research.
• GINO
• GrWin Graphics Library
• JAPI (Java Application Programming Interface)
• MATFOR
• ogpf
• PLplot
• Intel Visual Fortran for Windows.
• Winteracter
• LibGD, via the fortran-gdlib bindings.
• ...

See also the Graphics, plotting and user interfaces list of packages on the Fortran-lang site.