I wasn't sure if there was another thread like this, but I think it's time to have a place to keep track of content for GOG games that are only available from other sites.

Dying Light - Harran Military Rifle DLC (Requires Gemly account)
Little Nightmares - Tengu Mask DLC (Requires Bandai Namco account)
Little Nightmares - Fox Mask DLC (Requires Bandai Namco account)
War for the Overworld - Charity DLC - Humble Widget

In-Game Content that requires 3rd Party Account:
Hybrid Wars - Ivan - Requires Wargaming.net account

Other:
[url=https://www.gog.com/game/xenonauts_2_demo ]Xenonauts 2 Demo[/url] - Available on GOG, but not advertised in catalog

Let me know if you spot anything else or if there's any mistakes, and I will keep this list updated.

Hybrid Wars - Ivan is unlockable only with wargaming.net account.

Don't even know if that even works anymore given that Hybrid Wars is abandoned.

Xenonauts 2 beta is (or was at least) only available via clicking on a link from PC Gamer or somewhere off site.

You can grab it here:
https://www.gog.com/game/xenonauts_2_demo

Are you sure about the dying light content? I got quite irate about it on another thread, but when I looked at my account there was a 1.5gb patch which said it also contained all that dlc content. It doesn't show up as updated in the filter, but go into the download page and you should see a patch.

I guess the War for the Overworld Charity DLC also counts: http://buy.wftogame.com/charity/

Okay, I've added War for the Overworld, Hybrid Wars, and Xenonauts 2, I'll just separate them by little sub-categories for now. As more content is added, I'll figure out ways to organize them better.