The difference between these probabilities is zero, so Adv(A0) is zero. The same thing applies if we always return 0, or always return 1: the probability is the same for both F and G, so the advantage is zero. This adversary can't tell F and G apart.
The difference in probabilities is zero, implying that there is no advantage for either choice (F or G). This adversary cannot differentiate between F and G.