Guns, Germs and Steel is definitely not the best title for this book, but it's probably responsible for a considerable amount of its success; the title sounds exciting, militant and apocalyptic. However, the content of the book hardly focuses on guns, germs or steel at all; I suspect the title was chosen specifically because it is more exciting than the content.
The majority of Diamond's argument focuses on geography; if I were to be snarky about it, I'd title this book "Geography, Geography, and Geography", since Diamond seems to think this is the only variable that influences human development.
A more generous name might be "Geography, Grains and Genocide", if we're keeping with the G-alliteration theme. A more complete picture of Diamond's theory is that geography has a direct effect upon the agricultural productivity and political variety of a region, and this is directly responsible for the dominance of one culture over another, leading to genocide, whether direct or indirect, cultural or physical.