By the fact that it was generated using GPT. Same way you would go about classifying something as e.g. not made with slave labour or made with a production process that follows environmental pollution rules. That you can't easily detect it from the end product is not necessarily an obstruction to legislation.

Not saying it should be happening, but if abusive misuse continues, it is likely to happen. Regulations could force labeling on content with punishments if mislabeled (content that makes sense turns to gibberish later on after you've wasted time on it looking for the main point). Flagging could be done by the community (HN style), etc.