That's practically Valve's entire catalogue other than Half-Life itself. They don't just buy the rights, though. They hire the independents that originally made the thing.

Although back in the day, Gearbox was actually a good dev. They did the standalone expansions for HL1 (Opposing Force and Blue Shift) and also made Borderlands. I think Randy is getting too involved these days instead of letting the talent in the company cook.