Silent Storm and Jagged Alliance 2 are probably the closest we got on GOG at a stretch. Unfortunately, Tactics-style games are a bit rare on the PC. I recommend picking up X-COM, be it the original or the remake. I have heard good things about Valkyria Chronicles, but I think that one is for PS3 or Xbox360. The 3DS and other handheld systems are your best bet for finding Tactics games. Let's see...

Ogre Tactics
Ogre Battle: Knight of Lodis (a sequel to Ogre Tactics, found on the GBA.)
Devil Survivor
Disgaea
Fire Emblem
Advance Wars
Valkyrie Profile: Covenant of the Plume
Shining Force

I recommend using Youtube to look at these games and go from there for what to pick up.


EDIT: Come to think of it, the upcoming Banner Saga might do the ticket. It is for the PC, and has a tile-based combat system that resembles Tactics Ogre. Also, the recent Avernum: Escape from the Pit might qualify.

Well, Age of Wonders is not 100% tactics, but the empire/city management is very simple. The game itself is just brilliant.

Not on GOG, but Telepath Tactics may be worth keeping an eye on. It's still in development but there's a demo you can try.

http://sinisterdesign.net/products/telepath-tactics/

Vandal Hearts. I'm a big fan of Tactics Ogre too. Also the GBA version of Final Fantasy Tactics is different (storyline) to the others so you might want to check that one out.

There really aren't any games like FFT on GOG. That ype of game is nearly exclusive to consoles. Aside from a few good recommendations above, maybe Fantasy Wars. It's still nothing like FFT, but it's a turn-based tactical game with levelling units and a linear story progression along a world map with simple choices in where to go next.

If you want games like FFT, you're going to need a PS2 or PS Vita or Nintendo DS or something. Disgaea (I prefer the first one, but the DS version, not the PS2) is an excellent way to sink 200 hours into a game at whatever pace you can spare. It's rather like FFT. But there's nothing on GOG that will scratch a FFT itch.