Elmofongo: Well he is still Gandalf the Grey, he has not leveled up to Gandalf the White yet ;)
Nergal01: True. Besides, Gandalf the White got his ass kicked by the Witch King in ROTK. Figures that he wouldn't stand a chance against Sauron all by himself.
If Gandalf used all his potential and power and not hold back, he probably can take on Sauron by himself, whether he wins though is up to Tolkien. Gandalf pretty much gave the Witch King an ultimatum at the gates of Gondor, and probably won't hesitate to unleash his Maiar powers on WK.
"In rode the Lord of the Nazgul, under the archway that no enemy ever yet had passed, and all fled before his face.
All save one. There waiting, silent and still in the space before the Gate, sat Gandalf upon Shadowfax: Shadowfax who alone among the free horses of the earth endured the terror, unmoving, steadfast as a graven image in Rath Dinen.
"You cannot enter here," said Gandalf, and the huge shadow halted. "Go back to the abyss prepared for you! Go back! Fall into the nothingness that awaits you and your Master. Go!"
The Black Rider flung back his hood, and behold! he had a kingly crown; and yet upon no head visible was it set. The red fires shone between it and the mantled shoulders vast and dark. From a mouth unseen there came a deadly laughter.
"Old fool!" he said. "Old fool! This is my hour. Do you not know Death when you see it? Die now and curse in vain!" And with that he lifted high his sword and flames ran down the blade.
Gandalf did not move. And in that very moment, away behind in some courtyard of the City, a cock crowed…. And as if in answer there came from far away another note. Horns, horns, horns."
The deleted scene in the ROTK film of Gandalf falling and staff breaking is kinda dumb. Good thing it's deleted.