Act 1 - G/W/T can be used to express iteratively aspects of an algorithm such that you can derive it without knowing or fully understanding it (algorithm triangulation: think GPS where each satellite is a constraint and you derive through iteration of each passing scenario the general algorithm).

Act 2 - is really about process rather than G/W/T (which is really just AAA, arrange, act & assert).

Act 3 - again process, G/W/T isn't the solution to domain understanding, conversations are. You should be continually communicating to between all stake holders to maintain a current domain understanding.

We wrote an article recently on the limits of BDD [1], G/W/T didn't really come up, there are other more glaring issues with BDD when it comes to systems that intersect mismatched understandings of the real world between experts and users. Unrealistic wants and goals are killer. Additionally we started writing a tool to attach metadata to scenarios (G/W/T) so you can capture technical details about things called SpecStack [2]

[1] https://endian.io/articles/limits-of-bdd/ [2] https://github.com/endiangroup/specstack