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, and the OpenMP Application Program Interface v2.5 specification.
In the future, the GNU Fortran compiler will also support ISO/IEC 1539-1:2004 (Fortran 2003) and future Fortran standards. Partial support of that standard is already provided; the current status of Fortran 2003 support is reported in the Fortran 2003 status section of the documentation.
The next version of the Fortran standard (Fortran 2008) is currently being developed and the GNU Fortran compiler supports some of its new features. This support is based on the latest draft of the standard (available from http://www.nag.co.uk/sc22wg5/) and no guarantee of future compatibility is made, as the final standard might differ from the draft. For more information, see the Fortran 2008 status section.
Additionally, the GNU Fortran compilers supports the OpenMP specification (version 3.0, http://openmp.org/wp/openmp-specifications/).