gfortran is the name of the GNUFortrancompiler, which is part of the GNU Compiler Collection (GCC). gfortran has replaced the g77 compiler, which stopped development before GCC version 4.0. It includes full support for the Fortran 95 language and is compatible with most language extensions supported by g77, allowing it to serve as a drop-in replacement in many cases. Large parts of Fortran 2003 and Fortran 2008 have also been implemented.
An experimental version of gfortran was included in GCC versions 4.0.x, but only since version 4.1 has it been considered user-ready by its developers. Development is ongoing together with the rest of GCC. gfortran forked off from g95 in January 2003, which itself started in early 2000. The two codebases have "significantly diverged" according to GCC developers.