The period of time where Zits inhabits the body of Gus is very powerful and represents a moment of change in the book. Gus' job as an "Indian Tracker" is to lead his fellow soldiers to a First Nations settlement so that they can attack the people they find there. When he sees a young soldier trying to rescue a boy from the carnage, Gus helps them get away. I think that what Alexie was trying to get across with this narrative is that warfare is never as simple or as "clean" as we might expect. Even in clearly divided attacks, like the American army attacking First Nations people, the two sides are not necessarily monolithic in their experience or attitudes towards the event. Both Gus and Small Saint are two characters who are on the side of the army but change their minds and their actions. Gus says that he joined the army because he wanted to help people, and that is what he does. Alexie has used this character to give us a very powerful look into the fact that people may feel forced to do things they don't really want to do, especially in wartime.
One must wonder whether Gus' actions were entirely his own or whether Zits interfered in his helping Small Saint and Bow Boy. If Zits was able to influence Gus' actions, why couldn't he do so in any of the other bodies he was sent into? Perhaps because, deep down, Gus wanted to help people, Zits was able to steer him in this direction.
Alexie made some important literary decisions in choosing which character he wanted Zits to inhabit during each scene. I think that he chose characters who he could craft a narrative of morality and conscience around. When we read and talk about historical events, whether it be just days or hundreds of years ago, some of the reality of the situation is lost. We can fall prey to no longer seeing the actors and events as having been composed of real people with thoughts and feelings. Sometimes it just seems like stuff that happened, us versus them, black and white. In truth, even the most horrific of acts in history were committed by people with thoughts and feelings who must have negotiated with their sense of morality.