In order to be incorporated, a company has to have stock. Private companies still have shares even if they aren’t traded on public exchanges.

It’s possible that Gabe owns 100% of Valve’s stock, but it’s also possible that he’s sold some to other people or entities. Originally when Valve was founded, it was a split between Gabe and Mike Harrington, but Harrington reportedly sold his shares to Gabe when he left.

It’s also very likely that Gabe owns shares in other corporations, even just as personal retirement investments. But that’s not what I was talking about.