Spending a discounted sum of money for digital goods is not for the likes of GOG. No, they also have to do something weird with their sales. You can pick from a selection of encheapened indie games in this, the Indie Piñata sale, you can also have a whack at a mystery indie for $3.
Go to the GOG front page (opens in new tab) and you'll notice a dangling piñata, and a button to buy it. Do so, and you'll be given a random indie game worth between $10 and $45. I did so, for science, and got a copy of sci-fi social media simulator Redshirt.
GOG says the regularly discounted indie games and the potential piñata pool contain separate games, so your mystery surprise won't be something that's also on sale. If your indie collection is sparse, it seems like a good way to get at something you might not otherwise have bought. Otherwise, I do imagine there's a risk you'll be rewarded with something you already own.
In the sale itself, notable discounts include Hotline Miami 2 for 33% off, Papers, Please for 50% off, or Doorkickers for a whole 75% off. New deals and piñata possibilities will be added in just under three days.