GOG is Steam's main competition, and the only other big player in the game consolidation/management arena. There's Uplay and Epic Games and others, but Steam and GOG combined probably make up 99% of the market share. Unlike secondary/authorized sellers like Humble Bundle, Fanatical, GMG, Gamestop, etc, GOG is a primary store. Just like Steam is the primary store for the games Valve sells and hosts on its servers, GOG is the same. So there should be no hesitation in ordering from them when it comes to legitimacy. And as far as reputation, GOG is highly reputed for its stance on gaming (anti-DRM), the quality of its games (I tend to buy older games on GOG instead of Steam since the latter commonly sells games that don't work properly, whereas GOG makes a real effort to get them to work and guarantees it), and its customer service, which I personally haven't had to test but I've seen nothing but good reports about it. GOG Connect is also nice, as it lets you periodically add games you own on Steam to your GOG library, giving you a DRM-free version. The only downsides to GOG are that the sale prices are rarely as good as those on Steam and its resellers, though that's the price to pay for DRM-free copies and to support what seems to be a much better company, and the Galaxy client, while not bad, is very basic and not even close to the Steam client. And it pains me to say that, since the Steam client is atrocious.