Two different genres. GSG is like Victoria, 4X is like Age of Empires. Stellaris is a rare example of a game which blends 4X and GSG together, but it's the only time Paradox development studios has ever touched the 4X genre.
Obviously Stellaris works really well as a game, so there's something there but idk how you would make colonialism socialist without the benefit of outer space terra nullius.