What if Gandalf the White killed the Witch King?

[Zigûr]

Originally Posted by Moonraker

New leadership would have been necessary to re-inforce the will of Sauron, and I could imagine Sauron ordering the Mouth of Sauron into the fray to restore order in the ranks of the legions of Mordor.

Except that to the best of our knowledge he was hundreds of miles away (by the main road) in Barad-dûr at the time, so I don't see how he could simply be "sent into the fray."

We also have no knowledge about his skill as a military commander or even as someone to scare the Orcs into submission. He was "more cruel than any Orc," but that's about the extent of it.