Default gcc for linux

Why gcc@5.5.0 is the default version for gcc on linux?

We build the bottles for Linux on Ubuntu 16.04, and the version of GCC on Ubuntu 16.04 is GCC 5.5. It’s best if the version of the gcc formula matches the version of GCC used to build bottles. When we update the distribution of Linux used to build bottles, we’ll also update the version of the default gcc formula on Linux.

There’s a more detailed discussion of this topic here: https://github.com/Linuxbrew/brew/issues/679

I can’t use some libraries because they were built by an old gcc.
Example : Openblas should be built by gcc@9 to be used by gfortran@9.

I’m afraid I’m not at all familiar with Fortran. One possible way forward would be to recompile openblas using gcc@9. If you do that, to prevent breaking all other formulae that depend on openblas, you may want to make a private copy of the formula named say openblas-gcc9 and the brew install --cc=gcc@9 openblas-gcc9. If you go this route, let me know if it works out for you.