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 between these probabilities is zero, so Adv(A0) is zero. The same holds true if we always return 0 or always return 1: the probability is identical for both F and G, resulting in a zero advantage. This adversary cannot distinguish between F and G.