What kind of build? The most they'd need to do is to repackage it. And really GOG do it on their own, so all they need to do is to provide the game to GOG. There is no build involved for such older games.

> Patches also means compatibility with future operating systems, drivers, and hardware that you can't possibly know anything about today.

Those don't depend on distributors, they are universal. I.e. if they are doing it on Steam, they can use it on GOG as well.