Potentially a collaboration/society of existing employees. That would be the best alternative for those hoping to keep Valve as the Valve we already know.

Because his son is completely uninterested, he's entirely focused on racing cars, which if fair enough, but likely means he would simply sell the company given the chance.

Honestly, Valve should just become a worker-owned co-op when Gabe is gone. They already have the culture for it with their "desks on wheels" structure, where employees can move around the company to whatever project they are most interested in/can use their skills the most. That's usually one of the biggest hurdles to a company succeeding as a co-op, and Valve has been running that way for years.

They should be training AI on Gaben so one day it can take over Valve.