Deadlands: The Weird West is an alternate history setting for tabletop roleplaying games strataegy games, and fiction. In it, a covert war is being waged against demonic entities in the American West in the late 19th Century. It combines elements of the western, steampunk, horror, and fantasy genres. It should not be confused with Deadlands: Hell on Earth, Deadlands: Lost Colony, or Deadlands Noir. Hell on Earth is a late 21st Century post-apocalyptic variant of the setting. Lost Colony is set on an alien planet, in the same time period as Hell on Earth. Deadlands Noir is set in the early to mid 20th Century.
Deadlands: The Weird West as an RPG released with a custom rules system, sometimes called Deadlands Classic. It was subsequently converted to d20 (as Deadlands d20), GURPS (as GURPS Deadlands), and Savage Worlds (as Deadlands Reloaded).
The GURPS version was created for the Third Edition rules, but can be converted to Fourth Edition without much trouble, with the help of the GURPS Update conversion guide. The magic system can be used as-is, since it is a version of the default GURPS magic system modified to fit the setting. Harrowed or Blessed characters could be confusing to convert, however. Players can get help on the Steve Jackson Games GURPS Forum if needed.