Who do you think Diamond's audience is in general for Guns, Germs, and Steel?
This book is written for an audience that is not really expert in any field, but which is at least interested in the questions that Diamond is posing.
In this book, Diamond touches on issues that come from academic fields ranging from microbiology (how and why germs cause the illnesses they do) to political science (how different types of political organizations emerge). This means that the book cannot really be aimed at people who are expert in any given field the way a book could be if it really just touched on one subject.
The only real prerequisite for reading this book (other than being able to read at at least a high school level) is interest. You have to be interested in why the world has turned out the way it has and, specifically, why Europeans and their descendants came to dominate the world.
So this is a book aimed at anyone with a decent level of education and an interest in the major issue that the book explores.