"Gherkin is the language that Cucumber understands. It is a Business Readable, Domain Specific Language that lets you describe software’s behaviour without detailing how that behaviour is implemented." [1]
The example detailed how foldl is implemented.
Also, as it is intended to be a DSL for *business*, I think it has nothing to do with Haskell (the *technology*), i.e. no need for a "hs" in the file extension, just call it "whatever.gherkin" and pass it to a Gherkin-interpreter or something.