I recently committed an XSL stylesheet that allows us to transform the GuideXML documents (both guides and handbooks) to DocBook. This isn't part of a more elaborate move to try and push DocBook instead of GuideXML for the Gentoo Documentation though (I'd rather direct documentation development more to the Gentoo wiki instead once translations are allowed): instead, I use it to be able to generate our documentation in other formats (such as PDF but also ePub) when asked.

If you're not experienced with XSL: XSL stands for Extensible Stylesheet Language and can be seen as a way of "programming" in XML. A stylesheet allows developers to transform one XML document towards another format (either another XML, or as text-like output like wiki) while manipulating its contents. In case of documentation, we try to keep as much structure in the document as possible, but other uses could be to transform a large XML with only a few interesting fields towards a very small XML (only containing those fields you need) for further processing.

For now (and probably for the foreseeable future), the stylesheet is to be used in an offline mode (we are not going to provide auto-generated PDFs of all documents) as the process to convert a document from GuideXML to DocBook to XML:FO to PDF is quite resource-intensive. But users that are interested can use the stylesheet as linked above to create their own PDFs of the documentation.

Assuming you have a checkout of the Gentoo documentation, this process can be done as follows (example for the AMD64 handbook):

$ xsltproc docbook.xsl /path/to/handbook-amd64.xml > /somewhere/handbook-amd64.docbook
$ cd /somewhere
$ xsltproc --output handbook-amd64.fo --stringparam paper.type A4   
 /usr/share/sgml/docbook/xsl-stylesheets/fo/docbook.xsl handbook-amd64.docbook
$ fop handbook-amd64.fo handbook-amd64.pdf

The docbook stylesheets are offered by the app-text/docbook-xsl-stylesheets package whereas the fop command is provided by dev-java/fop.

I have an example output available (temporarily) at my dev space (amd64 handbook) but I'm not going to maintain this for long (so the link might not work in the near future).