Matlab was not able to detect g-fortran, since it only looks by default at the Intel's compilers. (please, see below)
>> mex -v timestwo.F
Verbose mode is on.
No MEX options file identified; looking for an implicit selection.
... Looking for compiler 'Intel Visual Fortran Composer XE 2013 with Microsoft SDK 7.1' ...
... Looking for environment variable 'IFORT_COMPILER14' ...No.
... Looking for environment variable 'IFORT_COMPILER13' ...No.
Did not find installed compiler 'Intel Visual Fortran Composer XE 2013 with Microsoft SDK 7.1'.
... Looking for compiler 'Intel Visual Fortran Composer XE 2013 with Microsoft Visual Studio 2013' ...
... Looking for environment variable 'IFORT_COMPILER14' ...No.
... Looking for environment variable 'IFORT_COMPILER13' ...No.
Did not find installed compiler 'Intel Visual Fortran Composer XE 2013 with Microsoft Visual Studio 2013'.
... Looking for compiler 'Intel Parallel Studio XE 2015 for Fortran with Microsoft SDK 7.1' ...
... Looking for environment variable 'IFORT_COMPILER15' ...No.
Did not find installed compiler 'Intel Parallel Studio XE 2015 for Fortran with Microsoft SDK 7.1'.
... Looking for compiler 'Intel Parallel Studio XE 2015 for Fortran with Microsoft Visual Studio 2013' ...
... Looking for environment variable 'IFORT_COMPILER15' ...No.
Did not find installed compiler 'Intel Parallel Studio XE 2015 for Fortran with Microsoft Visual Studio 2013'.
... Looking for compiler 'Intel Parallel Studio XE 2015 for Fortran with Microsoft Visual Studio 2015' ...
... Looking for environment variable 'IFORT_COMPILER15' ...No.
Did not find installed compiler 'Intel Parallel Studio XE 2015 for Fortran with Microsoft Visual Studio 2015'.
... Looking for compiler 'Intel Parallel Studio XE 2016 for Fortran with Microsoft SDK 7.1' ...
... Looking for environment variable 'IFORT_COMPILER16' ...No.
Did not find installed compiler 'Intel Parallel Studio XE 2016 for Fortran with Microsoft SDK 7.1'.
... Looking for compiler 'Intel Parallel Studio XE 2016 for Fortran with Microsoft Visual Studio 2013' ...
... Looking for environment variable 'IFORT_COMPILER16' ...No.
Did not find installed compiler 'Intel Parallel Studio XE 2016 for Fortran with Microsoft Visual Studio 2013'.
... Looking for compiler 'Intel Parallel Studio XE 2016 for Fortran with Microsoft Visual Studio 2015' ...
... Looking for environment variable 'IFORT_COMPILER16' ...No.
Did not find installed compiler 'Intel Parallel Studio XE 2016 for Fortran with Microsoft Visual Studio 2015'.
... Looking for compiler 'Intel Parallel Studio XE 2017 for Fortran with Microsoft SDK 7.1' ...
... Looking for environment variable 'IFORT_COMPILER17' ...No.
Did not find installed compiler 'Intel Parallel Studio XE 2017 for Fortran with Microsoft SDK 7.1'.
... Looking for compiler 'Intel Parallel Studio XE 2017 for Fortran with Microsoft Visual Studio 2013' ...
... Looking for environment variable 'IFORT_COMPILER17' ...No.
Did not find installed compiler 'Intel Parallel Studio XE 2017 for Fortran with Microsoft Visual Studio 2013'.
... Looking for compiler 'Intel Parallel Studio XE 2017 for Fortran with Microsoft Visual Studio 2015' ...
... Looking for environment variable 'IFORT_COMPILER17' ...No.
Did not find installed compiler 'Intel Parallel Studio XE 2017 for Fortran with Microsoft Visual Studio 2015'.
... Looking for compiler 'Intel Parallel Studio XE 2017 for Fortran with Microsoft Visual Studio 2017' ...
... Looking for environment variable 'IFORT_COMPILER17' ...No.
Did not find installed compiler 'Intel Parallel Studio XE 2017 for Fortran with Microsoft Visual Studio 2017'.
Error using mex
No supported compiler or SDK was found. For options, visit
https://www.mathworks.com/support/compilers.