To use the GNU Compiler Collection's gcc, g++, or gfortran with MATLAB MEX, first register the compilers by copying the provided GCC XML files to the appropriate location in MATLAB_R20XXX.app:

$ cp g*_maci64.xml /Applications/MATLAB_R2020b.app/bin/maci64/mexopts/

Note: The provided XML files are for gcc-10 and g++-10. If you would like to use a different version of GCC you must modifying these files accordingly.

Next, start MATLAB and check the value of $PATH to make sure it includes the location of your version of GCC:

>> getenv('PATH')

If you have GCC installed in a location such as /usr/local/bin (which is the case if you installed via Homebrew), then you may need to add this location to MATLAB's $PATH. This is best done by adding the following line to MATLAB's startup.m script:

setenv('PATH', ['/usr/local/bin:' getenv('PATH')])

The compiler files shipped with MATLAB may take priority over GCC by default. To fix this, either copy the provided Clang XML files to the appropriate location in MATLAB_R20XXX.app:

$ cp clang*_maci64.xml /Applications/MATLAB_R2020b.app/bin/maci64/mexopts/

or edit the Clang XML files in /Applications/MATLAB_R2020b.app/bin/maci64/mexopts/ to have Priority="B".

If all goes well, executing mex -setup C should select GCC (and give you the option to choose a different C compiler):

>> mex -setup C
MEX configured to use 'gcc-10' for C language compilation.

To choose a different C compiler, select one from the following:
Xcode with Clang  mex -setup:/Applications/MATLAB_R2020b.app/bin/maci64/mexopts/clang_maci64.xml C
gcc-10  mex -setup:/Applications/MATLAB_R2020b.app/bin/maci64/mexopts/gcc_maci64.xml C

Similarly, for C++:

>> mex -setup C++
MEX configured to use 'g++-10' for C++ language compilation.

To choose a different C++ compiler, select one from the following:
g++-10  mex -setup:/Applications/MATLAB_R2020b.app/bin/maci64/mexopts/g++_maci64.xml C++
Xcode Clang++  mex -setup:/Applications/MATLAB_R2020b.app/bin/maci64/mexopts/clang++_maci64.xml C++

Finally, for FORTRAN:

>> mex -setup FORTRAN
MEX configured to use 'gfortran' for FORTRAN language compilation.

(Note that if you have ifort installed, you may see an option here to select it.)