Currently, the simplest approach is not to try to write a GUI in Fortran, but to use another programming language. The best approach depends on whether you just want a nice dialog for generating parameters, or if you need plotting, or if are trying to actually do interactive real-time calculations.
For input generation, you could use a script language, like Tcl/Tk, to generate input data for a Fortran executable which is run separately, and which can plot results when the Fortran code is finished.
ISO-C-BINDING is an upcoming feature, being implemented in many Fortran compilers (i.e. Intel 10 beta). This will allow for for efficient, portable interfacing to C librarties as a standard language feature, and will likely become the normal approach to working with GUI and other non-math programming. I have had good results experimenting with interfacing Fortran to FLTK. In the near future, I expect to have some good Fortran/FLTK examples. It is probably better to look to this for long-term plans, but not really good for rith now, unless you want to delve into C code and experimenting with bleeding-edge Fortran features.
