KurtzD: Some good and logical points in here, and externally you can link games to your Steam profile to enjoy the benefits of what that entails... in a lot of cases this is not so true. To the person (human acknowledged) that stated my outrage was mis-placed.. maybe my crime was misplacing how much benefit I could get from GoG and what I thought was possible.
Linking a game to Steam that you didn't have a Steam key for doesn't give you any benefits of having bought the game on Steam, though. (Such as achievements.) Surely you knew that. There aren't any stores where you can buy DLC for a game you bought on a different store, so it's odd you'd think GOG would be different, plus I'm unclear why you'd even want this. If you already have the base game on Steam, why wouldn't you just get the DLC there too? It's not like there would be any benefit to having GOG DLC with a Steam game even if it was possible.
Attached is another example of how DLC is handled, in this case Sunless Sea. As you might infer from the file sizes, the Zubmariner DLC download is actually the entire game with the DLC included (so there's no point downloading the base game if you have Zubmariner). If you could use GOG DLC by itself without owning the base game, then buying the DLC would give you the full game + DLC for the price of the DLC alone, which you'd have to admit is unfair. This is the way the developers handled it; GOG didn't make them do it like that, so again, misplaced outrage. It's just not technically feasible for GOG to offer DLC without owning the base game.