Don't be fooled by the generic title, GURPS Fantasy is a world book. There is very little (only two small chapters) in the way of general rules about fantasy in the book. The rest is history, geography, and politics of the world of Yrth, written specifically for a Game Master. It works best if you let the players roam where they will, and use the book's info to be prepared for what they encouter. I really like the world itself, it's richly laid out and realistic; close enough to Middle Earth to be familiar to players, but different enough to keep them on their toes. However, such a detailed world is of little use in a hack-and-slash campain. For generic fantasy, use GURPS Magic, GURPS Fantasy Folk, and/or your own imagination.
GURPS Fantasy is good place to start a campaign, if the GM lacks the time or talent to build a world from scratch. There's enough detail to provide a lot of room for campaigns with a lot of different 'flavors'.
The background material can be used to generate adventure seeds, and make the world seem more "real" to the players.
For those that don't want a generic "high fantasy" setting, this might not work. It does have some nice twists - like Goblins being a viable character race, not a "cannon fodder" monster.