Most of Valve's money comes from taking a 30% cut of all sales through the Steam store, which is a lot of money. They're not abusing their market position as much as they could (e.g. if you buy a Steam key from anywhere else, like a key in a physical box at a physical store, or another online key retailer like Humble Bundle, Valve gets no money, as the publisher can generate as many Steam keys as they want for free), but it's the surplus value of the labour of employees of other companies that Gabe Newell accumulates.