On
www.gog.com, you have a menu across the top of the page. On that, if you're signed in, you'll have an ACCOUNT item, and if you pull that down, you'll see LIBRARY. Go to LIBRARY, and you'll get a bookshelf with all your games, including Witcher 3. Click it, and you'll get a pop-up that looks like this:
You can download direct from that page, you can click TRY GOG GALAXY to get the GOG Galaxy client, or you can pull down MORE to get links to the GOG Downloader.
If you block popups, though, you probably won't get any of that.