Short answer: nothing. Just as the first atmospheric nuclear tests mark the end of the era of natural C14 dating, the introduction of GPT and other advanced AI language models marks the end of being able to assume free-flowing natural language text has a human origin. Thus begins the age of Artificial Inanity. (See Neil Stephenson's Anathem for the origin of that phrase and an interesting take on the effect.)

Longer answer: there may be a period of some months still before models approaching the complexity of GPT3 are available at scale to truly black-hat actors like the worst of SEO firms. Training such a beast is still an expensive endeavor and so there is some gatekeeping yet.