I think that your example largely shows that it's not useful to make prediction markets about GDP for policies that have effectively no impact on GDP.

If HN wanted to run a prediction market for features, they should use a measure like user growth, or average score of posts that use the feature or something. Something directly related to the feature in question.

But presumably there are policies that have an impact on GDP, right? A prediction market there is going to function properly. If you want to know what tax rate or pandemic policy the government should implement, it will absolutely have an impact on GDP.