Gog.com(formerly Good Old Games) is part of CD Projekt Group - the publishers of the fantastic Witcher series of games. GOG started off as a firm which (re)distributed classic games without DRM and ensuring they run on modern systems.
Off late, they have evolved into digital distribution - and they’re going against the likes of Steam (and to much lesser extent, Origin). Most digital distribution platforms tend to push their own brand, force you to use their clients and lock you down to their own platform - so not having all your eggs in the same basket is a good thing. On the downside though, you need to have each of their own clients and you end up with a fragmented userbase and friendlist.
GOG.com’s attempt to gain more people use their digital distribution platform is novel - something they call GOG Connect. Basically, you link your Steam account to GOG.com and GOG.com will transfer games to your GOG library, to be available permanently. Obviously not all games are available - the games that can be transferred are subject to publishers/developers' participation and even with that, you can transfer them only for a short period of time.
This is an interesting approach - GOG knows that a consumer won’t buy a game again(unless you’re feeling overly generous) - so they take swallow a bit of loss with the assumption that these consumers will buy from GOG’s storefront in the future. Will this work out for them? I really doubt so. But hey, at least I know that bunch of my purchased games are available to be played from elsewhere, without having to repurchase or pirate them.