Thanks for the link and notice about the deal!
The process is a little vague as I don't run the GOG Galaxy software so here's what I ended up having to do.
First go to the gwent page, download it. The installer is a combo of the GOG Galaxy software which once it's installed will prompt you to install Gwent. You don't actually have to do the full 2GB+ download, instead just start it the download and that will put the game in your library from there (you can verify it through the website), then just pause/cancel the download.
The link in this thread should now allow you to Subscribe and get the game free through a button. That'll open the witcher store page and after a few moments the price will adjust to $0.00 and you can add it to cart and check out.