requirements for Garrysmod

So i was thinking about buying Garrysmod and i needed a source and it looked like there was 3 games. Do i have to buy all three games or just buy one of them?

GMod needs at least 1 Source game to work.

If you don’t have any, I recommend getting The Orange Box. It’s cheap and huge.

Alternatively, go here for a list of all Source games that will allow GMod to work.

Atleast Half-Life 2 or any other Source game