nrd224: Never heard anyone mention this website/platform in my 10+ years using steam.
Registered: Nov 2014 ... huh. Interesting.
Why cant Nvidia just give me a steam key like every other game redemption I've got?
Because Nvidia had a promo deal with CDPR, parent company of GoG.com. It was not a secret, and was prominently displayed even before you purchased your video card. I know, because I also bought a new video card and got the deal, and I checked the offer beforehand to make sure it was a GoG key and there's a big honking GoG.com logo right there, and not a Steam logo in sight.
Why do I have to download another gaming platform just to play this game? and if I don't download the platform why is the game in 7 different files and a patch that I have to manually download?
Because internet connections are rarely solid and a hiccup 30 gigs into a 40 gig download, forcing you to start all over would be a right pain in the ass.
In your 10+ years of using Steam, maybe perhaps you've come across the feature to add a non-Steam game to your library? Download Witcher 3 from GoG, install, add to your Steam library. Easy peasy. And quit complaining about getting something for free.