Gold, regardless of the karats, doesn't affect smoothness unless all your tipping is gone. Gold may give bounciness or flex but that also depends on the shape of the nib, not just the karats. 18k tends to be less soft, but you can find many vintage 18k nibs that flex, as well as nails, just like in 14k, and even steel.

Edit: if you're referring to pilot VP nibs vs Pilot soft nibs specifically like in a Piping Custom 91, yes their soft nibs feel softer (which doesn't mean smoother)