Sorry, I’m not sure I understand where you’re getting your limitation on GOG and expansive interpretation on Valve.
GOG’s says a court order that “specifically entitled someone to your GOG personal account” is enough. Arguably a will that leaves “my personal GOG account,” recognized by a court determining estate, would suffice. Why wouldn’t it?
Conversely, Valve is specific that Steam accounts “cannot be transferred via a will.” Not only is Valve affirmatively denying a will qualifies, it seems Valve is likely relying on an interpretation that the account is not descendible in the first place.