The package mingw32-gcc does not enable fortran language and thus does not provide i486-mingw32-gfortran.
Is this on purpose, or because no one needs fortran support ?
The package mingw-w64-gcc from AUR passes --enable-languages=c,lto,c++,objc,obj-c++,fortran,ada
What's the best move here ?
Ask kindly maintainer for fortran support in community package / create an AUR package mingw32-gfortran ?
The best move is to use mingw-w64-gcc from the AUR. Gets you all you ever wanted and lots more ;-) The more users this has, the faster someone will move it to the repos.
There are binary repos for the packages if you don't want to spend time building everything from source.