What's the relevance of the Gates Foundation? Gates didn't sell Microsoft in order to start it, he used his own personal fortune (fine, Warren Buffet helped, but he didn't sell any golden geese to afford it, either).
Gates sold a decent amount of his Microsoft stock to fund the foundation, something Gabe can't really do given the current structure of Valve. They would have to go public for him to do so, sell to private concerns, or something like that all actions which would dilute his control of Valve.
