You can currently get The Witcher 3: Wild Hunt for free on PC if you own the game on PS4/Xbox One.
All you need to know is download GOG Galaxy 2.0 and connect your PSN or Xbox Live account (whichever account you own the game in).
GOG Galaxy 2.0 is actually worth downloading if you game on PC regardless. It allows you to connect all of your game clients as well as PSN/Xbox Live accounts in order to see your entire library complete with achievements/trophies and friends lists for all platforms.
If you already own the game in your GOG Library, you can take advantage of the offer and get a free giftable code of the game that is valid for one week.