king3pj :
I also own the Witcher 3 on GOG. I just checked the Steam Store page and they have the words "This content requires the base game Witcher 3 on Steam in order to play" highlighted in purple.
Just out of curiosity I added it to my cart and tried to check out. When I got to the last part before confirming my payment method I got this message.
"Your transaction failed because you are trying to buy 'Witcher 3: Wild Hunt - Expansion Pass' which requires ownership of 'The Witcher® 3: Wild Hunt'. Please correct the error and try again."
The only thing I can think of is that the same checks are NOT in place when a game or DLC is bought as a gift, especially if the recipient doesn't have a Steam account. My brother gifted the season pass to me last Christmas, and I'm just now getting around to trying to figure out what to do with it. One thing I noticed is that when I look at the season pass in my Steam inventory, it does say that it's "tradeable". Could I trade it in for, say, an XCom 2 season pass?
I know that the Witcher 3 is DRM-free but I was hoping to do it completely above board, and to avoid downloading nastyware and not have any issues with updates/patches.