The book Guns, Germs, and Steel is not directly related to identity and culture in modern societies, though it can give us some hints about those things. What the book is more directly related to is the issue of power today, although not even that connection is completely direct.
Guns, Germs, and Steel is meant to explain why white people (Europeans and their descendants) came to be richer and more powerful than non-white people in the modern world. In that way, the book is related to the issue of power. The book tells us that white people in general became more powerful because of geographic luck. They originated from a land mass that was well-suited for the development of agriculture. Since agriculture developed there first, so did civilization and technology, thus making the white people more powerful. To this degree, Guns, Germs, and Steel does tell us why some societies today are more powerful than others. However, the book is much less able to tell us why some European societies are more powerful than others or why South Korea and Singapore, for example, were more economically powerful than China for decades. The book does not allow us to understand more fine-grained details such as that.
As for identity and culture, we can really only infer a connection. We know that the book tells us, in general, why some societies became rich and powerful while others did not. We can infer that power, or the lack thereof, helps to create societies’ identities and culture. In other words, the fact that New Guinea did not become rich or powerful surely has an impact on the way that New Guineans perceive themselves and on their culture. In this rather indirect way, we can say that Guns, Germs, and Steel is related to societies’ identities and cultures and not just to their power.