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,...

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.