My guess is that Jared Diamond would have preferred to have named his book something other than Guns, Germs, and Steel. However, I imagine that he gave the book this name because it hints at the answer to the major question posed in the book and because it is catchy.
In this book, Diamond is attempting to answer “Yali’s Question.” Yali, a native of New Guinea, asked Diamond (on p. 14)
Why is it that you white people developed so much cargo and brought it to New Guinea, but we black people had little cargo of our own?
Because of Yali’s question, Diamond set out to determine why Europeans and their descendants were so rich and powerful while people from other countries generally were not. The title of the book gives us a hint as to why this is. The title tells us that the “white people” were able to dominate the world because they had technology (guns and steel) and because they carried infectious diseases (germs) that decimated native populations elsewhere in the world. These factors allowed them to dominate the world. I imagine that Diamond accepted this title in part because it gives the proximate causes of European dominance.
However, I would think that Diamond would have preferred a title that really conveyed his main point. Diamond does not think that it is enough to say that Europeans dominated because they had guns, germs, and steel. Instead, Diamond spends most of the book trying to determine why the Europeans had these things while others did not. As a good academic, Diamond might have preferred to name his book something like “Geographic Luck: Why Europe Came to Dominate the World.” However, I imagine that he was willing to go along with Guns, Germs, and Steel as the title because it was catchier and more likely to sell books.