I think __GFORTRAN__ is undocumented - at least I could not find any .texi where it is documented. I think it should be documented in doc/cpp.texi, possibly also in fortran/*texi.
Patch for cpp.texi: http://gcc.gnu.org/ml/gcc-patches/2007-09/msg00533.html For fortran/*texi, see PR 31547.
Subject: Bug 33303 Author: burnus Date: Fri Sep 7 07:33:26 2007 New Revision: 128229 URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=128229 Log: 2007-09-07 Tobias Burnus <burnus@net-b.de> PR fortran/33303 * doc/cpp.texi (Common Predefined Macros): Add __GFORTRAN__. Modified: trunk/gcc/ChangeLog trunk/gcc/doc/cpp.texi
FIXED.