Hickory: And how would you install the extracted DLC files into Steam so that both the game and Steam recognises that they've been installed?
timmy010: you don't need to install. steam games, while they do create uninstall items, doesn't require installation. steam isn't even a player considering it should be drm-free. again, all theoretical
You are right. Any DRM-free designated game, by their publisher/developer, does not need its .exe to be authenticated by any client, prior to or at its launch. And that means that ,the Witcher 3 series does not need any DRM/client, including Steam, to run prior to or along with its titles. Their .exe can be run by their installation file just by double clicking it.
(Don't take the troll seriously by the way).
Also if someone actually notices the registry, installing any of these games through GOG Galaxy or Steam, does not create any registry entry under their respective client, that compels it to run strictly through it ('RUNNING' entry for Steam). It ony 'ties' them to the client for installation path and version recognition, serving shortcut boot commands and updating.
heavyforbidden1: If I pick up the 2 expansions from GOG but I've got the base game on Steam, can I manage to play em?
I suggest installing the main game from Steam. It should be the latest version of the game (1.31) and its installation will be registered on your PC's registry.
Download the Backup Installation of the expansion packs (free DLC and latest updates/patches are already provided by the Steam installation) from GOG.com/GOG Galaxy and run them. The Witcher 3 installation registry entry should be detected and the expansion installation will proceed as normal. Logic dictates that there is no reason for this not to work, provided both installations are of the same version (1.31), which they are.
I will run a relevant test and let you know soon enough.