http://www.pcgamer.com/spend-15-in-the-gog-halloween-sale-and-get-tales-from-the-borderlands-free/

When it comes to seasonal sales, Steam always makes the biggest splash, but that doesn't mean that it's the only game in town. GOG is holding some Halloween festivities of its own, and it's got a little something extra in the bowl, too. Anyone who spends at least $15 in the sale will get a free copy of Tales From the Borderlands, and there's also a pile of new horror-themed games available in GOG Connect. 

GOG Connect, which went live last summer, enables you to add games that you already own on Steam to your GOG library. There are some restrictions, and not all games are available at all times, but if you own, for instance, Stasis or The Last Door on Steam but not GOG, you can just go to this page, click the green check mark on each of them, and you're done—you now own them on GOG too. (There are 29 games eligible for GOG Connect during the sale, those are just a couple I happen to like.)