mqstout: Of all the "should be there" features -- it clearly shows someone with a strong intent to buy -- and it's not there. It's one at a time.
But implementing this wouldn't always work and would require scrutiny the way GOG works. Sometimes the "bundles" and "special editions" and whatnot don't mark the DLCs owned. (This is rarer than the other direction, which is always true, where owning all the contents of an edition of the game doesn't mark that edition as owned.)
Yes, it may not always work depending on implementation. There could be stuff like owning Witcher 3 and Witcher 3 GOTY but not Expansions for WItcher 3 ... obviously I would not care for the Expansion on Witcher 3 base game.
But stuff like that I could filter out manually or just ignore. It would be just nice to have a list which I can go through if I have something missing or not (so I might buy it before it vanishes from the store or whatsoever).
Going through a list of like 1000 games one by one is just too much for that purpose.
mrkgnao: They're not that far away from it.
About midway down the main GOG page, there's a section called "Recommended DLC's". It's essentially a selection of what you're asking for (and we all want). So the relatively hard part is already implemented --- they just need to expose the entire list, not just a selection.
That's actually not bad, sad that we can't get the full list. 3x 4 entries is not a lot considering most will be "soundtrack dlc" or something like that.