I hope Gabe has setup Valve in such a way that they can pass on his mentality as a whole inside the business practices themselves. I think, after all these years, he must have surely thought about what leaving would look like for Valve. Considering this is a guy who seemingly thinks in decades, I feel maybe even optimistically calm about it.
I'm a huge fan of the OSS model of keeping your core business fully unrelated to OSS but allowing and encouraging the use and contribution to OSS by people on your payroll because it really is a rising tide effect. There are just too many stories of a cool project becoming a company only to eventually reverse-robinhood the project into a closed source for-profit product.
From what I see it seems like the culture of the company is shared between the leadership roles so it might be possible for the company to continue doing as it has been doing after Gabe.
I think the people at valve are smart and they understand their business and the company very well and that this issue is being taken seriously too.
Good governance exists, it's just that for most companies there's not really an interest in having that because it gets in the way of personal interests of people that are already entrenched in power.