Sorry for the confusion, we're offering as many products as possible to ensure parity with the game on other Storefronts.
For clarity's sake WFTO was released on the 2nd of April 2015. This is the base game package, the Underlord edition was released alongside it.
Heart of Gold the first DLC expansion was released roughly one year later in May 2016, Crucible (A Free Game mode) in December 2016, My Pet Dungeon a paid DLC Game mode was released to celebrate the game's second anniversary on the 3rd of April 2017.
There are also a number of seasonal worker skins that were released between October 2015 and April 2017. These are included in a single package to keep the number of products on GOG down.
In addition to the products listed above there's also the Anniversary Edition, which is the base game, Underlord edition content, Heart of Gold, Crucible, and MPD at a slightly lower price than buying everything seperately. Likewise the Underlord Edition itself contains the base game and UE content at a lower price than buying individually.
All this content has been released in addition to over 50 free patches during the game's lifespan so far and we've still got more to come!
We've worked hard over the last couple of months to bring WFTO and all it's content to GOG so that our fans can finally experience the game and it's DLC without jumping through the hoop that is steam.
Hope that helps explain the situation.
Note: this is more directed to the GoG team as it pertains to properly marking items as "owned" here on GoG ;)
Considering that the Underlord Edition is included in the Anniversary Collection, should it not be marked as "owned" when buying the Anniversary Collection? As it is I could technically still buy the Underlord Edition while having bought the Anniversary Collection.
Additionally, for the GoG team as well, there is room to improve future sales when it includes several editions of the same game, buy clearly presenting the fact that some editions are included in others (e.g. a hierarchical tree): as it was it was not easy to identify which of the 9 packages to buy to get everything while not buying the same thing several times ;) (missed the worker skin collection in doing so, might be for later then)