The essence of Guns, Germs and Steel is to explain societal developments of different human communities with regards to their environment and geographical locations. The author sought to establish reasons as to why certain groups were able to grow and dominate different areas of the world and how they were able to succeed in their practices.
On the issue of the Bantus and their dominance of Africa versus the Pygmies and Khoisan who remain sparse, the author attributes the Bantus success to agriculture, labor specialization and technology development. The author asserts that societies become stable when they begin to settle down, engage in cultivation and domesticate their livestock.
The Bantus are known to practice agriculture while the pygmies and the Khoisan are known to be hunters and gatherers. These differences seem to favor the agricultural communities such as the Bantus who are able to increase their population and establish stronger leadership systems which eventually become empires. These agricultural groups enjoy advantages of labor specialization and this enables them to efficiently and effectively defend and conquer more territory since they can focus on weapon development and use.
You can answer this question by looking in Chapter 19. The answer to the question is very much in line with Diamond's argument in this book as a whole. In the book, Diamond argues that those societies that have agriculture tend to be much more powerful than those that do not. This was the case with the Bantu. They had food production, growing millet and sorghum, which allowed them to have higher population densities and more technology than the pygmies and the Khoisan had. These advantages allowed the Bantu to spread out and dominate Africa.