3G's isn't much, we have roller Costers that do much higher than that.
Tower of Terror 6.3G South Africa (Johannesburg)
Shock Wave 5.9G USA (Texas)
Detonator 5.5G UK (Surry)
Revolution 4.9G USA (California) 1976
Don't get me wrong, I would love to see it added. Mostly because it would add the Coriolis effect, and development has already said probably not.
I don't think the goal of the Galacticraft mod is to be extra realistic. I admit it would be nice for roleplaying (I don't know) and realism. If you prefer a more realistic mod, I suggest the advanced rocketry mod. I think galacticraft is more unique thought (the planets stands out more)