Well, the suggestion to read the novel having been made, I'll take a swat at giving a few extra points:
For one thing, Gandalf is immensely powerful, but -- being one of the faithful among the Maiar rather than a follower of Melkor/Morgoth (Sauron's master), he is under strictures the enemy is not. In Tolkien's words: "for those who will defend authority against rebellion must not themselves rebel" (The Silmarillion, p. 66). The Valar and Maiar must, as much as possible, work within the natural order to interfere as little as possible with the "free will" of the inhabitants of Arda. To use his power regularly not only taxes him within the circles of the world, but also would increase the danger of domineering the will of those around him; something which Morgoth, Sauron, and the Witch King certainly have no qualms about because their character is to dominate by fear and oppression, whereas those on the opposite side are there more as guardians and guides, to protect to some degree, but not to interfere with the free choice given to the Children of Iluvatar.
In addition, Gandalf (as one of the Maiar and therefore having been since before the making or marring of the world) know the dangerous temptation of power -- this is why he reacts as he does when Frodo offers him the Ring; he knows he could be corrupted:
Quote:
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'No!' cried Gandalf, springing to his feet. 'With that power I should have power too great and terrible. And over me the Ring would gain a power still greater and more deadly.' His eyes flashed and his face was lit as by a fire within. 'Do not tempt me! For I do not wish to become like the Dark Lord himself. Yet the way of the Ring to my heart is by pity, pity for weakness and the desire of strength to do good. Do not tempt me! I dare not take it, not even to keep it safe, unused. The wish to wield it would be too great for my strength. I shall have such need of it. Great perils lie before me.'
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(
The Fellowship of the Ring, Chap. 2: "The Shadow of the Past".)
In addition, Gandalf knew that the Witch King couldn't be slain "by hand of living man"; yet he stood up against him without (apparently) any fear -- it was the unexpected shift in fortune, which indicated that Sauron's plans were going awry, which called the leader of the Nazgūl away... and a lucky thing for Faramir, too, else he'd have been burned to death in the funeral pyre his father set for them both.
In the end, though, when you do read
LotR, read it
carefully; Tolkien put an awful lot of thought into all of this, and there are layers within layers within layers of subtleties going on there -- which is one of the reasons the book has lasted so long, and continued to grow in popularity as well as critical repute, and why it can bear many re-readings and still present you with surprising new facets each time.