The GNU Fortran compiler implements ISO/IEC 1539:1997 (Fortran 95). As such, it can also compile essentially all standard-compliant Fortran 90 and Fortran 77 programs. It also supports the ISO/IEC TR-15581 enhancements to allocatable arrays.
In the future, the GNU Fortran compiler will also support ISO/IEC 1539-1:2004 (Fortran 2003), ISO/IEC 1539-1:2010 (Fortran 2008) and future Fortran standards. Partial support of the Fortran 2003 and Fortran 2008 standard is already provided; the current status of the support is reported in the Fortran 2003 status and Fortran 2008 status sections of the documentation.
Additionally, the GNU Fortran compilers supports the OpenMP specification (version 3.1, http://openmp.org/wp/openmp-specifications/).