Nible1: Off-topic a little, but won't the Steam version of Cyberpunk also be released DRM-free?
Or does CDPR actually throw Denuvo on it / force a Steam login to start the game if bought on there?
To OP:
No, you could just add the GOG version of the game to your Steam library (I am sure you are aware of that feature, but just thought I should mention it). You just wouldn't be able to earn Steam achievements that way or track your game time on your Steam profile; you'd earn GOG Achievements and track it on the GOG client.
I (and probably everyone here) would urge you to buy on GOG to support DRM-free gaming, but...
... Hey, it's your money... You can support the idea that all games should be chained to a client ... That's fine...
:( Sad face
Witcher 3 was DRM free on Steam (game could run without Steam client running), so I expect the same for Cyberpunk. Unless of course CDPR does something under the hood we can't see.