Today I checked my library and noticed that the complete edition of this game, and the free DLC program, are available as standalone downloads. But I've never bought it on GOG or Steam or any other PC gaming platform. I do own the game on PS5, but I doubt that has anything to do with it. I do also have Witcher 1/2 enhanced in my GOG library, they were the last Witcher games I bought for PC.

You can try contacting one of the GOG employees from the forums (the Blues) and see if owning the game on PS5 gave you the PC versions free. It might actually be the case since they seem to really be generous when it comes to giving free upgrades to their customers.
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I'm very sure this has nothing to do with my PS5 copy that I bought. My GOG and PSN accounts aren't connected in any way, so GOG has no way of knowing that I bought it there. But if they're giving it away for free then I cant be the only one, so I asked here. I definitely wont be contacting support, never look a gift horse in the mouth.

It's not on a giveaway afaik. If you do a search, it's still priced and not free, though some parts are on sale.

Check your order & settings page and do a search just in case. I know you said you didn't buy it, but even if they gave it for free, it should show on that page as a "Free" item.

All the free games I've gotten show up there. I think even some of the free upgrades showed up there, but don't quote me on that yet lol. So try that and you may get your answer.
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They apparently did: https://www.ign.com/articles/the-witcher-3-is-free-on-pc-if-you-own-it-on-ps4-or-xbox-one

I know I bought a couple Ubisoft games on the XBox store & they showed up in my Ubisoft launcher on PC. I think something like this is pretty cool.