Agreed, the design of GPT is to be indistinguishable from real text, hence you’re fighting a losing battle (unless you have millions to train a rival network). The only key may be in the way GPT interprets language, we are all aware of how it does not understand what it’s says. If you can find a way to target a pre-trained network at that you may have a chance but in theory by the nature of GPT you will loose.