In addition to the formal documentation that is installed by GCC, there are several other text files in the gcc subdirectory with miscellaneous documentation:
Notes on GCC’s Native Language Support. FIXME: this should be part of this manual rather than a separate file.
Notes on the Free Translation Project.
The GNU General Public License, Versions 2 and 3.
The GNU Lesser General Public License, Versions 2.1 and 3.
Change log files for various parts of GCC.
Details of a few changes to the GCC front-end interface. FIXME: the information in this file should be part of general documentation of the front-end interface in this manual.
Information about new features in old versions of GCC. (For recent versions, the information is on the GCC web site.)
Information about portability issues when writing code in GCC. FIXME: why isn’t this part of this manual or of the GCC Coding Conventions?
FIXME: document such files in subdirectories, at least config, c, cp, objc, testsuite.