Where to a apply a patch to the gcc compiler (gFortran) -

Where to a apply a patch to the gcc compiler (gFortran) -

i installed yosemite , gfortran won't compile. found patch, copied , saved .patch file, don't understand apply it. here explanation help message:

"running command while in gcc source directory patch -p1 < patchfile"

my question gcc source directory? have gfortran directory at:

/usr/local/gfortran

following comment of basile starynkevitch, trying fighting way through building of gcc compiler.

one question: how know have prerequisite software, indicated in instructions: https://gcc.gnu.org/install/prerequisites.html? having os x 10.10 , xcode installed, need worry getting of these?

gcc compiler-construction gfortran osx-yosemite