What if Gandalf the White killed the Witch King?

[Zigûr]

Originally Posted by Moonraker

Or would another leader step in, such as the remaining 8 Nazgul, or even the Mouth of Sauron?

Are we talking about at the Pelennor itself? Given that "Gothmog, Lieutenant of Morgul" (whoever that was) stepped in as the commander - albeit seemingly a rather incompetent one whose strategy was mostly just 'throw the reserves in' and still lost - in the course of events as they actually happened, he seems to be the most likely candidate.