Heuristics
Always write Gherkin
When should I formalise with the Gherkin script?
You should always write Gherkin for the examples that you got our of the discovery phase. Writing the examples as Gherkin get’s you new insights. You don’t need to use it to automate.
Context
When doing Behaviour-Driven Development we want to formalise our examples so we can possibly use these to automate them.