@jlconlin ,

On macOS , Xcode contains the tools needed to compile native code on the machine, include gfortran. So you do not need to additionally install gfortran  for macOS , and just install gfortran for Linux.

For example:

jobs:
  test:
    name: Test
    runs-on: ${{ matrix.os }}
    strategy:
      matrix:
        os: [ubuntu-latest, macos-latest]
    steps:
      - name: Install gfortran for Linux
        if: matrix.os == 'ubuntu-latest'
        run: sudo apt-get install gfortran
  - name: Check gfortran version
    run: gfortran --version