Quite a lot of SEO already uses simple word generation techniques. It isn't clear GPT-3 is an improvement there - human text recognition might not be whatever Google does.

It may be that Google's algorithms don't care at all how human-like the text is, or that their own recognition algorithm/NN (whatever they use) isn't fooled. Even if it is affected, Google has the money and corpus to build its own competing NN to recognize GPT-3 text.

While I have no doubts that they could build NN capable of recognizing GPT-3 text I believe that this would still pose a problem given the amount of content to be analyzed at the scale that Google deals with

I'm sure Google out of all entities could handle scale.

That said, there might be a different threat to Google. GPT-3 seems really useful as a search engine of sorts (with the first answer implementing the 'I'm Feeling Lucky' button). Tune it for a query syntax, and for getting the 'top X' results somehow, then we just need the web corpus and a basic filter over the results. We could have a very interesting Google competitor.