I think Gandalf the white probably could thrash the WK in his sleep. They
are on completely different levels of the power spectrum.
Come on...he sends Andurin bursting into flames when Aragorn was going to
attack him. He probably would have melted the Nazgul armour and all dead
in his tracks. Perhaps he couldnt kill actually him due to the prophesy
but he could have kept sending the WK back to be remade.
I think Gandalf the White was probably the second most powerful entity on
ME..not far behind Sauron but quite a distance from #3 who would probably be
Galadriel and then followed by Glorfindel (who the WK was afraid
of).