Hover over ACCOUNT in the top bar, then click LIBRARY. Click Witcher 3 (or any game) then click "MORE" and then "GOG DOWNLOADER LINKS".
Of course, you first need GOG downloader in order for these links to work. I'll link it to you in a moment.
http://www.gog.com/downloader You download, then install it and run it. Then login to your GOG account. And then click any link on the GOG Downloader Links page.
So in steps:
Step 1: Download GOG Downloader.
http://www.gog.com/downloader Step 2: Install GOG Downloader, and then run it.
Step 3: Login to your GOG account from GOG Downloader.
Step 4: From the GOG website, hover your mouse on ACCOUNT, then click LIBRARY.
Step 5: Click The Witcher 3, then click MORE, and then GOG DOWNLOADER LINKS
Step 6: Click the link to any content you want to download. On some browsers you might have to accept the usage of GOG Downloader to use this link
Step 7: The game is now downloading.
Hope I helped.