GNU Fortran (gfortran) is a GNU FORTRAN compiler which replaced g77 around version 4.0 of the GNU Compiler Collection. GNU Fortran is a Fortran 95 compiler which supports most (but not entirely all) of Fortran 77, as well as parts of Fortran 2003 and Fortran 2008.
gfortran started off as a fork of g95, not from g77. gfortran has since diverged from its g95 origins.
The project home page is here.