I think the gap between Law and Luffy is only slightly bigger than the gap that exists between Sanji and Zoro. Close fights but the victor would most likely have the upperhand most of the fight.
I think the law-luffy power gap is bigger but thats because a) I think Luffy is the single strongest of the worst generation rivalled only by kid and b) I think the power gap between Sanji and Zoro isn't much bigger than it was before. Zoro still has the edge but it's not as if he stomps sanji. Luffy on the other hand could one shot Law.