This scene is highly inacurate. Gandalf is the chief of the Istari, wisest of the Maiar and messenger of Manwë, King of Valar. The only being in Middle-Earth capable of defeating Gandalf at this point is Sauron, another great Maiar. In the books, the scene is totally different. The WK enters, Gandlaf stands still and pretty much ignores his ''fire'' sword. He is neither surprised nor worried at the Witch King's arrival. He is then forced to ride back and save Faramir. The Wk flees before that.
Of course we know that no "man" can kill WK, but Gandalf isn't a man. Even Gandalf the Grey would've cleaned the floor with WK. Here, the Witch King got the jump on Gandalf... something that -N E V E R- happened in the books, even when he was the Grey. In the books, Gandalf was WAITING for the Witch King - like a fucking boss. It's pretty disappointing. Peter Jackson IS a great director but I'm not sure why he made Gandalf look like a bitch in that scene....