Publicity stunt supported by a few publishers. It happened a week before their summer sale. If they do another round it will probably be in November before the winter sale. It could also be against Steam's terms of service. It could have hurt Gog, too. I stopped buying from them entirely after they did that stunt.
No reason it should be against Steam's TOS - it's just publishers giving free GOG versions of games if you have the Steam version. Akin to EA automatically registering Origin versions of games if it finds the Steam version on your PC. It very definitely could be an issue for their future sales though, considering a vocal number of their users double-dip on DRM-free versions of games they already have on Steam. I wouldn't be surprised if it quietly died out because of that.