This one. It looks like they're using GPT3 to translate the natural-language problem context and goal into a format called PDDL (planning domain definition language), then feeding the result into a separate program that generates a plan based on the context and goal.
With that in mind, the thing they're really testing here is how well GPT3 can translate the natural-language prompt into PDDL, evaluated on the basis of whether the generated PDDL can actually solve the problem and how long the resulting solution takes.
Naturally, I could be wrong but that's at least what it looks like.