I've said this before and I'll say it again. There's no incentive for people on GOG to keep playing. When servers were populated, almost every game was dominated by proxy spammers and disc spammers. Nobody knew how to play or wanted to learn. Of course people would stop playing the game, because nobody knew how to set up a server or how to play the game properly. Surprise surprise, the players who spammed the no-skill weapons didn't end up having a good time (because it took no skill to get kills) and the people who were trying to play legitimately kept dying cheap deaths. And when people tried to explain the problem, the spammers wouldn't stop because the weapons were enabled and the tactic was getting them kills.
As Skorp said, GOG also has zero community aspect. How is it possible to organize a game when you can't even add people as friends? I know you can have friends on vBulletin forums, why not on GOG? There's also no way of being able to tell who's good at this game, and who therefore knows more. I've seen a lot of players who have no idea what they're talking about saying things as though they're facts, for instance, that aliens are OP. Well, since nobody knows who anybody else is, everybody thinks that only they themselves are right and that nobody else knows as much as them. So I can say "You're wrong, aliens aren't OP," but who am I to them? Olde72 is just a name. GOG doesn't even have an "hours played" feature.
Now, there's also a very realistic answer for why so few people still play this game on GOG: it's because the game was released as a freebie to test out GOG's servers. Really, let's see it for what it really is, a way to test multiplayer servers. And people played it because it's kind of a novelty item. It's not a generic shooter; it's different enough to catch people's attention and get them to play for a bit. It's also in keeping with GOG"s trademark for supporting classic games. But those reasons aren't enough to hold people's interest when they don't know how to play, don't know where to turn to to improve, hardly know how to set up servers, and the client has barely any community. In fact, it may have never even been intended to breathe new life into the game. The way people consider the game on Steam versus how they do on GOG is extremely different. So the way I see it, the drop-off in players is not only understandable, not only predictable, but inevitable.