Well, some other games aren't released on GOG and probably never will be, but have it's own a DRM-Free version to buy, usually on producent webpage or other official distribution websites.
Of course there would be a best if Subnautica get GOG …
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Yes, Steam have a offline mode, but it's still a DRM, you still have to have a internet connection to (re)install game, even from local files data and you still can't make a sterling backup.