Use '/usr/local/opt/gcc@7' instead of '/usr/local/opt/gcc'
Homebrew uses gcc-8 for gcc and no longer gcc-7. To continue to use gcc-7 in homebrew, we will need to install gcc@7. gcc@7 will then be linked to '/usr/local/opt/gcc@7'.
Homebrew uses gcc-8 for gcc and no longer gcc-7. To continue to use gcc-7 in homebrew, we will need to install gcc@7. gcc@7 will then be linked to '/usr/local/opt/gcc@7'.