Question: If you complete the Dark Carnival campaign, and take the gnome all the way to the helicopter, on easy difficulty, do you still get the achievement?

Answer: 
On Xbox 360, you definitely do, yes. I can't see why the PC version would be different.
You can do it on any difficulty, whatever the support (PC/Xbox).

Since it isn't especially an easy one, if you just want to get the achiev', you'll probably prefer to get it on easy. But it's also a great challenge to try it on expert.