Why, according to Guns, Germs, and Steel, was China able to dominate East Asia?Was it because of the geography? I reread the section several times, but I can't seem to get the answer.
Yes, it was because of the geography in a couple of different ways.
First, China had a lot of different places where agriculture could arise. It had many places with a decent climate and it had many domesticable crops and animals. This allowed it to develop civilization in a number of areas in what is now China.
Second, China did not have any great barriers within it and it had long east-west rivers. These things allowed for a great deal of diffusion of culture. They also allowed for competition between civilizations. This led to a stronger culture and also, eventually, to a big and centralized state.
Because it had the crops and animals to develop civilization and because it had a topography that allowed diffusion and competition, China became strong enough to dominate East Asia.