To get The Witcher 3 for FREE, you need to download GOG Galaxy 2.0 and connect 3rd party gamng platform such as Steam, Origin, PSN and Xbox Live with The Witcher 3 on it. Once your integration is synced, to be able to claim a GOG copy of the game.

You can get the base The Witcher 3 or the Game of the Year Edition, depending on what you already own on a corresponding platform.

Important information about the giveaway 

• If you bought the base game on a qualifying platform, you can claim the base game on GOG.
• If you bought the GOTY version on a qualifying platform, you can claim the GOTY version on GOG.
• If you bought the base game and add-ons separately, you will only receive the base game on GOG. :/ 
• Owners of the Switch edition of The Witcher 3: Wild Hunt - Complete Edition won't be eligible to benefit from this giveaway (since there is no Switch integration on GOG Galaxy 2.0)
• If you already have The Witcher 3: Wild Hunt in your GOG.COM library, you can still take advantage of this offer and receive a giftable code of the game (either The Witcher 3: Wild Hunt or The Witcher 3: Wild Hunt GOTY Edition depending on the version you own) valid for a week.

You can read more about this offer in FAQ section on GOG.com.