The gfortran command supports all the options supported by the gcc command. Only options specific to gfortran are documented here.
See GCC Command Options, for information on the non-Fortran-specific aspects of the gcc command (and, therefore, the gfortran command).
All gcc and gfortran options are accepted both by gfortran and by gcc (as well as any other drivers built at the same time, such as g++), since adding gfortran to the gcc distribution enables acceptance of gfortran options by all of the relevant drivers.
In some cases, options have positive and negative forms; the negative form of -ffoo would be -fno-foo. This manual documents only one of these two forms, whichever one is not the default.