GURPS attempts to satisfy campaigns across pretty much every genre, in pretty much every time period. The standard basic characters/campaigns books, plus perhaps Magic, Fantasy, and Banestorm make life pretty easy for a fantasy GM, while the basic books plus perhaps UltraTech, Infinite Worlds, Space, BioTech, or Traveller Interstellar Wars make life easy for a sci fi GM. Until now, it has been a bit problematic running a campaign set between the civil war and near-tech cyberpunk.
High Tech addresses this time period. It concentrates on ironmongery, with real world examples of weapons and ammunition as used throughout history. It includes dates of introduction and wide reproduction for time travel games, and enough information for people generally unfamiliar with firearms to sound the part of their gunslinger characters.
It also has many, many pages of non-weapon/armor equipment, and frankly, that was the part I liked the most. My players enjoyed being able to look up a diving bell, a WW2 tank, and a napoleaonic-era timepiece.
For what it is worth, my minimal set of GURPS books is the main two, plus magic, ultra tech, and now high tech. With those, I can generally convert items from other systems or wing it, regardless of era.