Why log in to GOG.com or Facebook when game is on Steam?

[SpiderDragon]

This is so weird that steam game, why is it that it require other than steam?

[Lagrange]

To sync progress and collection across GOG, Steam, iOS and Android.

[Blue]

The game uses GOG's Galaxy platform for it's multiplayer and matchmaking but the main reason for that is more to do with the fact that CD Projekt, the Poland-based developers and publishers of GWENT are also the owners of GOG.com.

[乱馬 インバース]

Because GOG is:
1. Completely DRM-free and always will be. Thus it's automatically better.

2. GOG is CD Projekt's own product, so all of the deals on CD Projekt's own games and all related products (like soundtracks and etc) will always be better on GOG. Besides, by buying on GOG you're directly paying/supporting CD Projekt, so that's a very big point for GOG as well, while on Steam there's a lot of utter trash which isn't worth even a penny in many cases, and GOG has mostly cult classics and globally confirmed top tier games which by the absolutely largest part stood the test of time very well, so it's much easier to support GOG in a long run than Steam because GOG's almost completely devoid of worthless/scamming trash. Did I mention NO DRM?

3. When you get/download Gwent on GOG, you get a completely free copy of Enhanced Edition Director's Cut version of first Witcher game into your library, to keep forever and without any DRM. Steam makes you pay for first Witcher still. Yes, really. Even AFTER Gwent appeared on Steam.

4. Steam's client currently supports only Steam services, while GOG, with it's Galaxy 2.0 client, can support almost every other large digital distribution service out there, even including consoles. With Galaxy 2.0 you can control your libraries, achievements, and etc., all from one singular client, while other services make you install their own proprietary standalone clients each and every time.

[Oxyderkis]

It uses GOG Privacy and settings and you must login to GOG to agree with them.
That's the answer.