GOG is awful at DLC management. If you own the complete edition of a game, which includes many or all DLCs, these included DLCs still show un-owned on the store's pages. Keeping track of owned or missing DLCs is confusing because of that. It's an old problem, and GOG isn't interested one bit in looking into it.

I don't mind free DLCs to not be automatically added (it's quite satisfying to click on them and "buy" them freely, and al least they show as owned afterwards), but don't expect GOG to solve this given how they deal with DLC in general.

The base GAME doesn't show as owned either. And vice versa. If I buy ALL the components of a complete edition, the complete edition still gets pushed at me as a "recommended game". Like, no shit I'd like it. I already bought it!!

It's not consistent, it depends on each case, sometimes it works as it should and other times we get any of the different scenarios you all have been listing.

It's not just DLCs either. It can happen with entire games too. Specifically, game collections:

Buying Trine Ultimate Collection for example results in Trine 1-3 marked as owned but Trine 4 not :)

Oops sorry, yes that is true. ha ha ha

I had forgotten I just select 'hide all owned games', and because I own all the free ones, that then just shows me the DLCs and extras I don't have.

As you can see I misremembered slightly. :)

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As has been mentioned here, GOG's software or manual editing for all I know, leaves much to be desired, and certainly on Sales pages, we often see games listed that we know we own, but are not indicated as such. So I would not want to rely on them automating inclusion of free DLC into my library. Far better I manually check the free games page.

There are a lot of imperfections at GOG, but at least when you visit the store page for a game or DLC it shows as owned, and I don't recall it ever being otherwise. You also get warnings in the cart ... it is rather annoying though, not knowing beforehand.

That doesn't make any sense, as the large majority of those games don't have DLC. Let's say it's 200 to be generous, but even then you wouldn't "periodically visit" pages. You'd just check once, when you actually played the game. This also ignores paid DLC, which is most of it, and presumably you would want to know about that too. Actually free DLC that adds any significant content is pretty rare, and even then there's usually a reason it's DLC, or else it would just be part of the base game.

Agreed. Check before you start playing a game. Makes life simple.

Well, generally games don't have DLCs untill they do. Not every game on GOG premiers with all the DLCs available.
Apparently you check the game, download it play it and forget it - but that's not the only way. How about people who keep the offline copy of the library and want to keep it updated and complete? Again, this is supposed to be an option, you don't like it - don't use it.
And you find out which have DLCs by visiting all of them, because game could (and often do) have a DLC added after some time.

I agree with them not being auto-added. As mentioned above, sometimes they're language or HD texture packs. Or sometimes they're "real life seasonal" or "risky content reskin" items that people might not want in the game. With Galaxy auto-installing DLCs you have, that could burn some people.

But, of course, GOG is seriously silly by not having a "show me DLC I don't own for games that I do" filter.

Nope, 100% wrong. I only download offline installers, which I do when buying the game. I keep games updated by checking the blue dots in my library. No. This is not hard to understand; don't know why you're being deliberately obtuse. You visit the page ONCE. When you're ready to play the game.

And that's fine, but this is YOUR way of handling GOG games, not everyone's. Yep, it seems it's hard to understand that other people may be doing things differently than you. Again, that's what YOU do and I don't want to change it or judge it. I want to keep my offline collection complete and updated with the help of proposed OPTION (i.e. don't like it - don't use it).