I find it ironic that gollum – a system for creating documentation (it powers GitHub’s wikis) – has some of the worst documentation I’ve seen. They have

That’s it. So good luck if you want to do something ridiculously simple like use GitHub Flavored Markdown instead of vanilla.

Edit: After some searching, I found that someone had opened an issue about precisely this question. This was closed with the remark that gollum does not support “specifying a particular markdown rendering library” (which sort of disingenuously suggests that this is just a matter of choosing between different libraries for rendering a particular language, rather than choosing between different dialects of a language with different semantics). Subsequent comments do little to illuminate the matter. Things are made even more complicated by my discovery that wiki pages seem to be rendered with GF markdown, whereas the edit-mode preview uses vanilla. No bueno.