Connect to GoG via the launcher and enter your DLC code located on the back of the manual. You will be brought to the store front where the Free 16 DLC Program is available to buy for $0.00.
Once you own it, to download the DLC:
1. Go to Library in GoG client
2. Locate The Witcher 3: Wild Hunt
3. Click the down arrow on the game box and select "Download Extras"
4. Under the DLC Installers list, click the name of any DLC you would like to download and the download & installation will automatically begin.