gfortran-6-multilib software package provides GNU Fortran compiler (multilib support), 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-6-multilib
gfortran-6-multilib is installed in your system.
Make ensure the gfortran-6-multilib package were installed using the commands given below,
$ sudo dpkg-query -l | grep gfortran-6-multilib *
You will get with gfortran-6-multilib package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | Development |
| Package | gfortran-6-multilib |
| Version | 6.2.0-10 |
| Description | GNU Fortran compiler (multilib support) |
| Sub Section | devel |