gfortran-mingw-w64-i686 software package provides GNU Fortran compiler for MinGW-w64 targeting Win32, you can install in your Debian 9 (Stretch) by running the commands given below on the terminal,
$ sudo apt-get update $ sudo apt-get install gfortran-mingw-w64-i686
gfortran-mingw-w64-i686 is installed in your system.
Make ensure the gfortran-mingw-w64-i686 package were installed using the commands given below,
$ sudo dpkg-query -l | grep gfortran-mingw-w64-i686 *
You will get with gfortran-mingw-w64-i686 package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | Development |
| Package | gfortran-mingw-w64-i686 |
| Version | 6.1.1-12+19.1 |
| Description | GNU Fortran compiler for MinGW-w64 targeting Win32 |
| Sub Section | devel |