"We apologize, but for maintenance work the registration of new accounts is currently blocked. Please check back later." But it still says that so there probably is some maintenance going (wr)on(g).
GPU-Z can detect some reflashed GPUs, not sure to which extend. This in combination with your benchmark idea should do the trick.
Or you take it apart and look up the markings on the die.
By ChatGPT u mean Dall-E?