This is both simple and complex, but can be mostly simplified by keeping your own copies of the games you own, and passing on relevant account matters like password, username and email address.

Nothing to stop anyone having a joint account at GOG etc, though things get complicated when it comes to serials, especially for multiplayer.

Also issues regarding combining existing accounts, where more than one copy of the same game occurs. So best to leave separate and not burden GOG with extra work, that they may be unwilling to do.

As far as GOG are concerned, unless you claim otherwise, the owner of an account is the one who has the password, username and current email address ... all of which can be changed by that person. Any complexities lie with multiple accounts by same person, where any similarities may occur i.e. credit card.

GOG don't really have anything that points back to a specific person, that can't be owned or controlled by an inheritor.

So the only issue really lies with the required elements not being passed on.

You just need to accept and realize that you may not be able to combine or split an inherited account ... especially that last.