Quoting Mark Mitchell <m...@codesourcery.com>:
I don't understand the full situation with MELT. But, you cannot combine GPL'd and GFDL's stuff, so I don't think you can auto-generate GFDL documentation from GPL'd code on the MELT branch.
I don't see a problem if the auto-generated GFDL documentation is identical to documentation that has been previously released under the GFDL. Or if the only portions not covered by the above are generated from content for which the user of the autogeneration machinery has full authorship rights.
You could generate GPL'd documentation, though.
But then it must not rely on GFDL files; the GCC documentation generally uses some texinfo infrastructure.