Question: What source book provides rules for running games like GURPS Space in a fantasy setting?
Context: GURPS Mysteries is a source book for the "GURPS" Role-playing game. GURPS Space is a "genre toolkit" for creating Science Fiction campaigns using the GURPS role-playing game.  It performs a similar purpose as GURPS Fantasy does for Fantasy games.  Rules and guidelines are provided for running games from science fantasy and space opera to hard science fiction, creating worlds and planets and notes about aliens races.  The first edition was published in 1988. GURPS Infinite Worlds is a supplement for the Fourth Edition of the "GURPS" role-playing game, published by Steve Jackson Games in 2005 and written by Kenneth Hite, Steve Jackson, and John M. Ford.  It expands upon the campaign setting of conflict between the "Infinity Patrol", which is the time-travel agency on "our" Earth, referred to as Homeline, and "Centrum" across a multiplicity of alternate history Earths.  This was presented in the "Fourth Edition GURPS Basic Set" (and originated in the Third Edition supplements "GURPS Time Travel", "GURPS Alternate Earths", and "GURPS Alternate Earths II"). GURPS Bio-Tech is a GURPS, the Generic Universal Role Playing Game, sourcebook that covers the implementation of biotechnology in the game.  The first edition of the book was written for GURPS Third Edition, while the second edition of GURPS Bio-Tech was written for GURPS Fourth Edition.  Both editions of the game are primarily focused on providing supplemental rules, campaign material, and examples of the uses of biotechnology for the players and game-master alike.  The second edition contains two outlines for campaign settings ("Alexander Athanatos" and "Draconis") but is primarily focused on providing rules and examples of devices that Game Masters could adapt for use in their own campaigns. GURPS Magic is a source book for the GURPS Role-playing game from Steve Jackson Games that provides in depth coverage of magic in the context of GURPS.  The book expands on the material outlined in the Basic Set, provides alternative forms of magic for GMs to use, and contains much more material.  The first edition was published in 1989. GURPS Martial Arts is a source book for the GURPS role-playing game, published by Steve Jackson Games; the most recent edition was scheduled to be released in 2007.  "GURPS Martial Arts" includes new perks, skills, techniques, styles, weapons, and combat rules for GURPS, as well as history on the martial arts, pregenerated NPCs, and ideas for martial-arts campaigns.  The book is an essential for any game that features large amounts of melee combat, in any genre or setting. GURPS Fantasy is a "Genre Toolkit" source book for the GURPS Role-playing game.  The fourth edition of GURPS separates the fantasy parts into fantasy and a setting book called GURPS Banestorm GURPS Bestiary is a source book for the GURPS role-playing game system containing information and statistics of animals.  It also contains information animal player character templates, and tips for fitting animals into adventures.  The first edition was published in 1988. GURPS Banestorm, written by Phil Masters and Jonathan Woodward, was released in October 2005.  It is a setting sourcebook for the fourth edition of the GURPS Role-playing game.  It details a fantasy setting called "Yrth" that has been updated from the older GURPS Fantasy source books Orcslayer and GURPS Magic (first edition).  The standard fantasy elements such as Wizards, Orcs, Elves, and Dwarves are present, along with connections to Infinite Worlds.  There are also some more unusual fantastic races like the Reptile Men, and several others which can be added in as desired by the game master. d20 Past is a d20 based role-playing game released by Wizards of the Coast in 2005 as a supplement to "d20 Modern", providing a framework and new rules for campaigns set any historic settings from the Renaissance to World War II, including new character options and rules for early modern firearms.  It begins with an exploration of the historical period between around 1450 and 1950, then provides rules for creating characters and campaigns that blend realism and fantasy.  "d20 Past" also presents alternative campaign models, along with all the rules needed to play swashbuckling pirate adventures, Victorian horror investigations, thrilling Pulp Era escapades, and more, including three ready-to-play campaign modules.
Answer: GURPS Fantasy