Ask HN: How will Google fare in a web which is predominantly walled gardens?

Just fine, I presume. Google has several successful walled gardens of their own, I don't really foresee that changing anytime soon.

Fine. Many walled gardens let Google's crawler index their complete content - but show partial contents and paywalls to humans. This is quite common on subscriber-only news sites.

But it depends on exactly how walled you mean.