Can ChatGPT evaluate how good ChatGPT-generated output is? This seems prone to exaggerating blind-spots, but OTOH, creating and criticising are different skills, and criticising is usually easier.
Not General, but there's IQ tests. Undergraduate examinations. Can also involve humans in the loop (though not iterate as fast), through usage of ChatGPT, CAPTCHA's, votes on reddit/hackernews/stackexchanges, even pay people to evaluate it.
Going back to the moat, even ordinary technology tends to improve, and a headstart can be maintained - provided its possible to improve it. So a question is whether ChatGPT is a kind of plateau, that can't be improved very much so others catch up while it stands still, or it's on a curve.
A significant factor is whether being ahead helps you keep ahead - a crucial thing is you gather usage data that is unavailable to followers. This data is more significant for this type of technology than any other - see above.