I can\'t find any info on it, but only the other way around (e.g., how to set CMake to use clang).
I\'ve installed gcc-4.8 using brew, setup all dependencies, header
Just to add that there is also a CMake variable "CMAKE_Fortran_COMPILER" to pick up GNU Fortran rather than clang fortran. But it seems to be missing in the documentation
cmake -DCMAKE_Fortran_COMPILER=/usr/.../bin/gfortran-6.x.0