Edward_Carnby: tremere110, thanks for the info !
If somebody interested - I wrote a letter to Handy-games about JA:R GOG release & here is an answer:
"Hi
Thanks for your email!
It probably won’t come to GOG because it needs the Steam API for multiplayer and we probably don’t want to remove that feature for GOG.
But it is not a final decision made yet.
Thank you!
Bye"
It seems the game is really crap, so we're probably not missing anything special.
Maybe it will launch on GOG in a year or two, when devs really need the money.