Does ghost walk prevent the model from suffering the -2" when crossing the barrier?
A barrier is an obstacle, and the barrier special rule imposes a penalty on models advancing through it. So, putting it together, ghostly would allow a model to advance through the barrier (an obstacle) without suffering the effects of its barrier special rule (a penalty).
Generally ghostly is pretty strong, and, as long as you’re allowed to advance through something in the first place, you can ignore all of the bad stuff.