INSTALLING THE FREE CONTENT:
Here is how you can grab and install your new DLC in The Witcher 3: Wild Hunt. Add the 16 FREE DLC Program to your account from its product page. It's like buying a game, but free.
A) If you're using GOG Galaxy:
--Select The Witcher 3: Wild Hunt from the sidebar to open its game page.
--Click on the drop-down labeled "More", (located next to the PLAY button) and choose "Configure".
--The Configure page will feature all available DLCs listed towards the bottom.
--Select each DLC that you want to install, and GOG Galaxy will do the rest. If you want to remove the DLC from your game, simply uncheck the selection box.
B) If you're using the website:
You can download installers for each DLC pack manually. Simply head to the Library section of My Account, locate The Witcher 3: Wild Hunt, and download the files you want. Afterwards, run each one as if you were installing a game or patch.