Just a guess, but I think they might be talking about how some collections of games get a single subforum rather than each game getting its own dedicated space. Like how the gold box games include multiple games but each collection gets a single subforum for questions pertaining to all of the games in that collection, or maybe how something like The Longest Journey/Dreamfall/Dreamfall Chapters are all packed into a single subforum.

I think for some games they offer only one discussion section for all of them. Don't know why. Maybe so that users don't feel lonely if the game is not played much and there is not much activity.

They probably could break them up even more. Maybe even give up the sections and have a keyword/filter structure.

But then the forum coding looks quite hand-made and not very much up to date.
I also always wondered why GOG did this. Probably to prevent even more hierachy. But then tagging of threads would be the obvious way out.

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It started off as one forum for each series, but around the time new games started dropping in, things got a bit more confusing, so the forum for Divine Divinity, Beyond Divinity, Divinity: Original Sin, and Divinity: Dragon Commander is "Divinity Series", but Divinity: Original Sin 2 has its own forum.

I think the big problem is that anyone coming to the site would naturally expect each game's page to have a link to the discussion forum for that game. There actually is a forum for each game (aside from some of the oldest games which are per-series instead of per-game) but for whatever reason GOG doesn't put a link to it on the game page, hence the confusion.

You can look up the forum for any game from the "All forums" page that you can access through the "Community" menu on the navbar.

Alternatively, there's a browser script/extension that I made called Barefoot Essentials which, among other things, adds a forum link to every game's page.

My guess is they see the game card page only as a selling thing. The idea of having a game specific community which you can join after you bought a game is kind of lost. Steam solves this a bit better. They have a dedicated community hub page for many games (not sure if all).

Fortunately I'm using your script. :)