Gandalf Vs. The Witch King

[Raynor]

Gandalf would use his ring plus supernatural powers of a maiar; the Witch King only has his lesser ring & added demonic force to help counter the maiar. Gandalf has a formiddable elvish blade, a mightier steed, & likely as not a higher power potential to draw upon.

I will repeat, we have no clue how these abilities will be used, how they are countered, what is their effects on the opponent, if Gandalf and the witch-king would necessarily make the same decisions each and every time, etc.

Nothing warrants an automatic result; such a result is possible only if we would be dealing with a very, very simple situation. This is not the case; quite the contrary.
Edit:

Above all Gandalf prevented the Lord of the Nazgul entering Minas Tirith, & is therefore victorious overall.

He is victorious overall because of the whole "whole train of human resistance, that he himself has kindled and organized" (letter #156); there isn't just one cause to the departure of the witch-king, and if victory occured, it was due to mass organisation, not to personal martial abilties proved in combat.