Why did Gregory have this problem? Because he presupposes a real relation of God to the world. He presupposes that God is really determined in virtue of the world.
“Why can you say that? It is just the opposite! He holds that God is utterly transcendent. That we never contact him. Thus, the opposite of what you just said!”
No, but the matter is subtle. Here is the point. Gregory holds that if God were to effect something after not having effected it (“after” being once again taken from the perspective of imaginary time from the point of view of the flux of creation, not from God’s point of view as it were), then God would have to change. Since Gregory rightly holds that God cannot change he denies the antecedent.
But the conditional proposition itself is false! The conditional proposition itself is indebted to the following view of agency: That agent which effects something new must itself be really related to that something new. This premise will indeed land you in Gregory’s dilemma. Aristotle also held that premise. He held that premise and therefore concluded that the first effect of the Unmoved Mover had to be uniform. Were that first effect not uniform, the UM would have to change in order to produce a non-uniform effect.
But both of these views premise a real relation of God to the world, in the manner stated: No agent can effect something new without being really related to that something new.
We will some day pick up Aquinas on essence and existence and give the reasons for the intelligibility of holding that indeed God can effect something new without himself being affected.