In the MGN2MECH directory, the makefile is linked to the makefile.intel.
I would do a make clean to verify that there are no old *.o files or old executable files in the directory.
To use the makefile.gcc, I had to remove that link and create a new one.
rm makefile
ln -s makefile.gcc makefile
If you are on system that uses modules, then check to see what modules are available using
module list
load the gcc compiler using something like the following (dependent on what modules are available on your system)
module load openmpi_4.0.1/gcc_9.1.0
Then run make using the following command:
make |& tee makefile.log
Please attach the makefile.log file to this issue.