>Gamers are a very select group of people, and there's no reason Valve couldn't target them.
This is true, especially among the subset of gamers who build their own computers. They are particularly ripe for experimentation with Linux, and would represent the Innovator/Early Adopter stage in the TAL [1]. Get those guys on board and the rest may see it as new and cool and follow.
[1]: https://en.wikipedia.org/wiki/Technology_adoption_lifecycle*