Retail version of Witcher 3 comes with GOG.com activation code. You can play the game without activating the code, but you'll miss the updates and free DLCs and you won't be able to buy Expansions.

If you activate the code here on GOG, you get access to updates, free DLCs and goodies (artbook, soundtrack and stuff like that) and you could also buy Expansions. The game still stays DRM-free, you do not need any client or be online at all to play it. The Galaxy client is 100% optional, you don't have to use it, you can download the game through browser like any other files. Even if you install the game through Galaxy, you do not need Galaxy running to play the game.