EXACT SAME thing for me here.
I only have The Witcher 3 Next-Gen edition installed, while the "Standard" edition I only OWN, but haven't INSTALLED it.
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Ironically, when I googled this issue the other day, and then eventually the search's results landed me in THIS thread,
the latest reply here (the one by user Zanozza) was just 2 (TWO) minutes old !! :O
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Disappointingly, (but perhaps also somewhat EXPECTEDLY), the latest GOG update (which occurred yesterday around this time) did NOT address the issue AT ALL...
I tried several solutions, such as
* Reinstalling GOG (but not the game itself, obviously)
* "Hiding" The Witcher 3 "Standard" edition from my games list.
Didn't work.
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Interestingly, I DO have a THEORY of how and why this happens, but it's too in-depth to discuss here.
In short:
GOG deceives your Operating System by plugging in the file-base and system requirements for your "Standard" edition of the game.
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Typical post-CP77 CDPR... -_-
.