What particular period(s) does Guns, Germs, and Steel deal with?

This book deals with a number of different time periods,
but the bulk of the book is about things that happened in prehistoric times.  These
ancient events and conditions are the main drivers of Diamond's argument.  He is arguing
in the book that the most important aspect of human history is when food production
started in various places, an event that happened as long ago as 10,000 or 11,000 years
in the past.

Diamond also looks at more recent events,
however.  He looks at events that happened as late as the 1500s, when the Spanish
conquered the Incas, or even the 1800s, when the Maori conquered the Moriori.  This
book, then, is not confined to any one time period.  It looks at events in the distant
past and then discusses ways that they have impacted events that happened much more
recently.  In doing so, Diamond discusses events ranging from tens of thousands of years
ago to those that occurred less than two centuries ago.