I think GOG has it's own system and network for multiplayer games, TerraTech does not use that. I guess if the devs get enough sales from GOG they may look at supporting that too but I would not hold my breath.
As an aside has there been any mention of MP on the console versions? If they did say something I guess I forgot it, my life is rather full ATM so I would not be surprised if I read something and promptly forgot it.
Well as they're giving away skins with the Console version (free for first month) which only work in MP at a guess they do have it. BUT a dev chimed in on a previous thread (I forget who) the console MP isn't going to be cross platform. So how well MP is received will I think depend on how much uptake/price on the consoles.
As to the OP's question: I know you used to be able to launch the game with the TT executable (in the game folder), does that no longer work? I ran it like that for quite a long time until I realised that my play time wasn't then recorded in Steam.