Technically you can get by with just the Basic Set, which is two books: a player's book (mostly for character creation and some basic rules) and the game master's book (which goes into the mechanics more thoroughly and lists out a lot of optional stuff such as details from environmental hazards to how difficult it is for PCs to find jobs when not adventuring).
The problem is that I'd imagine a substantial number of players are looking to play a typical fantasy game though (certainly not all, but a lot), and to do that with the Basic Set requires a good bit of work for the GM. He'd have to come up with stats for a lot of monsters and a lot of equipment and maybe spells if they're in the game.
That's where the other books come in. They're mostly advice on how to do this sort of thing. The Fantasy book for example goes into a lot of detail on how to build a fantasy world, provides some templates for common critters and character types, provides some more spells, things like that.
It's all up to the GM how much effort he or she is going to put into coming up with stuff on their own. The books like Fantasy, Low Tech and Magic are huge helps, but not required.
Although it is still a bit telling that you could almost consider that 5 must read books to do a proper fantasy setting. Or you could skip them and use the Dungeon Fantasy PDFs to just do a proper D&D knock off, but it's still extra stuff.
It's the price you pay for it though. GURPS has rules for everything if you want it. Supposedly one of the books has rules on raising a biological spaceship as a zombie. No joke.