EverNightX: You can't expect an installer to know about installs from other platforms.
The installer should prompt for the location of my game install; that's common sense. Not expecting a copy sold by GOG. It shouldn't matter where I got my copy from as long as I give it a path with a valid game installation. It should care only about the folder I point it to having the necessary DLLs, executables and assets.
EverNightX: You also can't expect a DRM protected Steam version to accept content that is not from Steam.
Have you ever installed any mod ever? Did Steam complain about you running a mod on your Steam bought game? What you said makes no sense. Steam only cares that when I tell their launcher to run an executable, that I have the license for it; which I do (the main game license).
Grargar: "Please note that you are required to own Age of Wonders 4 on GOG.com to be able to
purchase and play Age of Wonders 4: Primal Fury"
Hmmm....
So it's not DRM-FREE.
THX1342: https://www.gog.com/en/game/age_of_wonders_4_primal_fury (as an example of AoW4 DLC) is sold as DRM-FREE;
but when I download the DLC it is an executable that demand's a GOG version of the game installed. This doesn't sound like it is DRM free, it is linked to a copy of the game bought strictly on GOG.
timppu: Cross-compatibility across different service/store versions of games is not what DRM-free is about.
You could just as well claim that because you can't run GOG games on PS5 or Nintendo Switch, they are not really DRM-free games.
It's not "cross-compatibility"; I want the executables and content for the DLC no questions asked. If there is an installer asking me to prove I own something else from GOG, it's not DRM free. Forget about Steam. I bought the DLC and I wan't it's content; what I do with it is my own concern. GOG uses a 3rd party installer that encrypts the contents of the DLC unless you prove to it you own another GOG license. This is DRM