Whenever the first noncreature spell of a turn is cast, counter that spell.
Nullstone Gargoyle cares about the first noncreature spell cast during a turn, not the first one it “sees.” If a noncreature spell was cast before Nullstone Gargoyle entered the battlefield, it won’t counter the next one.
If another spell is cast in response to the first spell, Nullstone Gargoyle doesn’t affect that second spell.
