WritingFantasy is a Twine story format designed by Sophie Houlden, with a similar aim to Gordian Book — outputting a Twine story as a gamebook. Gordian Book and WritingFantasy have many bits of shared functionality, but WF aims to produce an HTML output suitable for EPub, while GB is focussed on PDF output and export. You can use WritingFantasy as a proofing tool for Gordian stories, if you like, or any other workflow that makes sense.
You can find WritingFanatsy here.
Gordian Book implements WritingFantasy passage tags as synonyms for its own to allow a story to work in both formats, as follows:
1, to prenumber a passage