try this

Author: jacobwilliams

.github/workflows/CI.yml

@@ -49,6 +49,9 @@ jobs:
     - name: Install GFortran Linux
       if: contains( matrix.os, 'ubuntu')
       run: |
+        sudo add-apt-repository ppa:ubuntu-toolchain-r/test
+        sudo apt-get update
+        sudo apt-get install -y gcc-${GCC_V} gfortran-${GCC_V}
         sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-${GCC_V} 100 \
         --slave /usr/bin/gfortran gfortran /usr/bin/gfortran-${GCC_V} \
         --slave /usr/bin/gcov gcov /usr/bin/gcov-${GCC_V}