Why doesn't the GOG version of Prey have the multiplayer and will it be added later?

Because it uses steamworks and unlikely.

Thanks for the info! Guess, I'll buy it on sale; for steam.

You seem certain that this game's multiplayer still has a community playing it right now?

I played it 2 weeks ago, however I always played as human hunter no matter how long I've waited and what team I chose (human hunter or lovely spiders). The game is made on Unreal engine (not the CryEngine like the base game) and it s@cks. It's impossible to lose a game being a hunter, not fun.

I got it a while back for the single player here on GOG and finally got to it in my backlog and I am really enjoying it, game has got me a few times with the tension and occasional jump scare but I am really glad I grabbed it.