The G77 manual or "info g77" will tell you all you need to know about using g77.
Why is g77 currently not supported? gcc-4.0 changed the front end interface for all languages in gcc. No volunteer or company stepped up to update g77 to the gcc-4.0 infrastructure, so it was dropped from the source tree. The Fortran-95 front end for gcc was under active development, so it has taken g77's place as the fortran compiler that is part of gcc.