1m² of perfect black body in vacuum at 343K should emit about 700W, according to the Stephan-Bolzmann formula. Should do for a consumer GPU, or an H100.

Since the heat distribution over the radiator is not ideally uniform, I suppose that the emitted power will be less, if they want to keep the chips at 70C.

Since the temperature has the fourth power in the formula, having the GPU work at 100C would increase the radiator efficiency quite a bit.