Issue with installing gsynth

[athenachu1005]

Thanks for the great package first.

I use install.packages("gsynth", type = "source") to install the package and got the warning as follow,
ld: warning: directory not found for option '-L/usr/local/gfortran/lib/gcc/x86_64-apple-darwin18/8.2.0'
ld: warning: dylib (/usr/local/gfortran/lib/libgfortran.dylib) was built for newer macOS version (11.5) than being linked (10.13)
ld: warning: dylib (/usr/local/gfortran/lib/libquadmath.dylib) was built for newer macOS version (11.5) than being linked (10.13)
installing to /Library/Frameworks/R.framework/Versions/4.1/Resources/library/00LOCK-gsynth/00new/gsynth/libs

and so when I use the function gsynth, it shows the following error,
Error: package or namespace load failed for ‘gsynth’ in dyn.load(file, DLLpath = DLLpath, ...):
unable to load shared object '/Library/Frameworks/R.framework/Versions/4.1/Resources/library/gsynth/libs/gsynth.so':
dlopen(/Library/Frameworks/R.framework/Versions/4.1/Resources/library/gsynth/libs/gsynth.so, 6): Library not loaded: /usr/local/lib/libgfortran.2.dylib
Referenced from: /Library/Frameworks/R.framework/Versions/4.1/Resources/library/gsynth/libs/gsynth.so
Reason: image not found
do you know what is the issue here?

thanks for the help in advance.