Welcome to the GOG forums!
I warmly recommend you
Sid Meier's Alpha Centauri!
It is IMO the best 4X game GOG has to offer - and there are plenty of good ones, here.
Consider it like a "Civilization in space" kind of game, with loads of new techonlogies (all developed from real scientific theories, so that it does seem as realistic as possible), units, conditions of victory and factions.
That game is so incredibly well made, reactive and complex... it could hook you for months!
Also, you could be interested in
Master of Orion. Two great games for only 5,99$!
Edit: I admit I am a bit biased towards these games... I always thought galactic empires are the pinnacle of awesomeness, so a game that allows the player to create one is just a must-buy for me! :)