`gf' from Gary Houston is short for "general formatter", i.e., it can work on documents which use the ISO "general" document type definition (DTD). It can convert SGML documents conforming to a small number of DTDs into various output formats: LaTeX, ASCII, RTF and Texinfo. However not every output format can be generated for every DTD.
Apart from the general DTD, gf supports the HTML DTD used in the WWW project and Gary's Snafu DTD. `gf' is not intended as a flexible system for hacking up a formatter for a random DTD, but as a usable document production system for a few DTDs.