I know it is not a GOG Game, but I think Sword of the Stars is a great Space Sim that plays very well, has a very demanding AI, is Fun, and looks really nice.
It has two expansions that really add to the game: Born of Blood, and A Murder of Crows.
I do not think this encroaches on GOG as they do not have this type of game for sale as of yet.
Sword of the Stars Official Site

Point of order: Sword of the Stars is a 4X game, not a Space Sim. A Space Sim is like a flight sim, but in space (like Freespace). It is, however, a great game. I need to get those expansion packs sometime. Also, to anyone who wants to pick it up, I would suggest gamersgate as they have a $30 pack with the original and both expansions. They are also the only place to buy AMoC as they paid to have it created, or at least thats what I understand.

Play Homeworld and Homeworld 2 or Sins of a Solar Empire. They are better than this game.

Apples and Oranges, man.

Those aren't space sims... they're good 4X/RTS games, but a space sim is like Freespace or something.

Who said they are space sims? He said Sword of the Stars and it is a RTS game, and he said it is great. And I suggest him Homeworld and Sins, because they are better RTS' than Sword of Stars? Is this wrong? Don't try to make me stupid, I know what is space sims and what is RTS games.

Except, of course, Sword of the Stars is not an RTS either. It is a turn-based 4X-strategy game with real-time tactical space combat. Granted, one could classify Sins as a 4X-strategy game as well, but as it's real-time it can not hope to compete with Sword of the Stars when it comes to depth and complexity.

The TS?
I've only played SoaSE and Homeworld my self, but Swords of the Stars looks somewhat interesting.

When I play a space game I simply must be able to blow things up, ideally in a spectacular fashion. I don't think space battles happen very often in real life. :)

Well, then you don't need a space "simulator" but simply a shooter :-P

I'd take it one step farther and say that space "sims" should at least have a proper flight model, which most of them do NOT (Freespace included). Independence War, Terminus, and Evochron Renegades/Legends seem to be the few exceptions...
...and, of course, there's Orbiter, but you can't blow stuff up in that.
(This is mostly because to me, the term "sim" implies "realism first".)

When you don't even know it's a turn-based game, a question 'did you even play it?!' comes in mind

I wanted to say strategy game, but wrote RTS by mistake. Yes, I played them all.

Good, I was almost afraid someone with case of uncovered arrogancy is running around GoG forums :D (After we got rid of last one so easily)