I have played most of the space games (MOO, MOO2, Reach for the stars, Imperium Galactica, Imperium Galactica 2, Space Empire IV, Birth of the federation, Star Wars: Rebellion and Pax Imperia: Eminent domain).
All this games are Space Empires games...and therefore some aspects will come in all the games (diplomacy, Espionage, Ships building, space events and so on). However all this games have proper aspects (for example in SEIV and Pax Imperia, you can travel only with Star lanes). I haven't played Galactic Civilization yet, still i can see from the game concept that it is different from the games i have played: ship movement and combat is different, Unit planets is deeper than what we had until now (MOO3 has something similar, but both games are new), more events than in the previous games, the idea of good or evil path, real options to different victories, internal politics, deep diplomacy that works (here again only MOO3 has something similar)
That's all for now...anyway, if i had to compare Galactic Civilization to another Space game, SEIV wouldn't be my first choice. I would say something between MOO3 and Birth of the federation