What source book provides rules for running games like GURPS Space in a fantasy setting?
GURPS Space:  It performs a similar purpose as GURPS Fantasy does for Fantasy games. GURPS Space:  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. GURPS Fantasy: GURPS Fantasy is a "Genre Toolkit" source book for the GURPS Role-playing game. GURPS Fantasy:  The fourth edition of GURPS separates the fantasy parts into fantasy and a setting book called GURPS Banestorm
GURPS Fantasy