Yeah, since Valve is not public the odds of the next head being an outside hire or some loud tech exec are extremely low.

GabeN is most likely to pick someone who's been around with the company since the early days, like Erik Johnson or Scott Lynch.

If he wants it to stay in the family, his own son Gray could take over, he's also a game dev.

Gabe has often expressed distrust of publicly traded markets, a plausible outcome is that his ownership gets converted into an employee-owned trust or a collective buyback, this would effectively permanently lock Valve into its current profile, distributing profils back into salaries and bonuses for the staff.