Fortran is developed by the Working Group 5 of Sub-Committee 22 of the Joint Technical Committee 1 of the International Organization for Standardization and the International Electrotechnical Commission (IEC). This group is known as WG5. Official Fortran standard documents are available for purchase from ISO; a collection of free documents (typically final drafts) are also available on the wiki.
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.
GNU Fortran also supports almost all of ISO/IEC 1539-1:2004
(Fortran 2003) and ISO/IEC 1539-1:2010 (Fortran 2008).
It has partial support for features introduced in ISO/IEC
1539:2018 (Fortran 2018), the most recent version of the Fortran
language standard, including full support for the Technical Specification
Further Interoperability of Fortran with C (ISO/IEC TS 29113:2012).
More details on support for these standards can be
found in the following sections of the documentation.
Additionally, the GNU Fortran compilers supports the OpenMP specification (version 4.5 and partial support of the features of the 5.0 version, https://openmp.org/openmp-specifications/). There also is support for the OpenACC specification (targeting version 2.6, https://www.openacc.org/). See https://gcc.gnu.org/wiki/OpenACC for more information.