Gandalf and The Witch King of Angmar never actually fought, their battle was interrupted. However, Gandalf as Gandalf The Grey did hold off ALL nine of the Nazgul for the whole of the night upon Weathertop. He was able to hold his own 1 Vs 9 including the Witch King and he only fled in order to try and hasten to Rivendel and get aid to Frodo. At the battle of Plennor the Witch King was enhanced by Sauron but Gandalf was by this point The White.
Gandalf the White said himself that the Witch-King was his equal or greater.
If Gandalf The Grey could hold off all nine Nazgul would the enhancement placed on the Witch King really be enough to outmatch all the Nines strength PLUS the additional strength granted to him by his "promotion" to The White? Sauron admittedly would be able to defeat the Balrog but to get to that conclusion by using Gandalf as a comparison may be inaccurate.
Gandalf said he did not defeat the Nine only drive them back and then he fled. Four went after and five did not. Nothing can stand against the Nine. And once more Gandalf the White himself said before the throne of Gondor that the Witch-King is his equal or greater and that he fears him.
'I galloped to Weathertop like a gale, and I reached it before sundown on my second day from Bree-and they were there before me. They drew away from me, for they felt the coming of my anger and they dared not face it while the Sun was in the sky. But they closed round at night, and I was besieged on the hill-top, in the old ring of Amon Sûl. I was hard put to it indeed: such light and flame cannot have been seen on Weathertop since the war-beacons of old.
`At sunrise I escaped and fled towards the north. I could not hope to do more. It was impossible to find you, Frodo, in the wilderness, and it would have been folly to try with all the Nine at my heels. So I had to trust to Aragorn. But I hoped to draw some of them off, and yet reach Rivendell ahead of you and send out help. Four Riders did indeed follow me, but they turned back after a while and made for the Ford, it seems. That helped a little, for there were only five, not nine, when your camp was attacked.
(TFotR (II) The Council of Elrond)
Sounds like Gandalf could only handle the Nine during the day and it was said that the Nine could not see in the light. Blindness gives Gandalf a nice edge. But when night came Gandalf had to try to defend himself and when the sun rose who managed to escape knowing that the Nine would have killed him.
Proof:
The Lord of the Rings: The Return of the King
Chapter IV: The Siege of Gondor Pg. 800-801 (One volume edition)
'Is Faramir come?' he asked.
'No,' said Gandalf. 'But he still lived when I left him. Yet he is resolved to stay with the reargurad, lest the retreat over the Pelennor become a rout. He may, perhaps, hold his men together long enough, but I doubt it. He is pitted against a foe too great. For one has come that I feared.'
'Not - the Dark Lord?' cried Pippin, forgetting his place in his terror.
Denethor laughed bitterly. 'Nay, not yet, Master Peregrin! He will not come save only to triumph over me when all is won. He uses others as his weapons. So do all great lords, if they are wise, Master Halfling. Or why should I sit here in my tower and think, and watch, and wair, spending even my sons? For I can still wield a brand.'
He stood up and cast opened his long black cloak, and behold! he was clad in mail beneath, and grit with a long sword, great-hilted in a sheath of black and silver. 'Thus have I walked, and thus now for many years have I slept,' he said, 'lest we age the body should grow soft and timid.'
'Yet now under the Lord of Barad-dur the most fell of all his captains is already master of your outer walls' said Gandalf. 'King of Angmar long ago, Sorcerer, Ringwraith, Lord of the Nazgul, a spear of terror in the hand of Sauron, shadow of despair.'
'Then, Mithrandir, you had a foe to match you,' Said Denethor. 'For myself, I have long known who is chief captain of the hosts of the Dark Tower. Is this all that you have returned to say? Or cab it be that you have withdrawn because you are overmatched?'
Pippin trembled, fearing that Gandalf would be stung to sudden wrath, but his fear was needless. 'It might be so,' Gandalf answered softly.