I found Some post on steam community where developer said that co-op is steam only so i assume, the devs implemented multiplayer mode with the use of steamworks multiplayer that require steam account and other steam exclusive features so chances we get online mode on gog are low. the devs would have to implement universal multiplayer specyfically written for the game to have multi on other platforms than steam, and steamworks multiplayer is much simpler and faster to implement than designing server/lobby hosting and browser in-game.
