[Knight of Gondor]

The Quest would have failed far earlier if Gandalf had not fallen.

Remember that, shortly after leaving Lothlorien (where he had been taken shortly after awakening on top of Zirak-Zigal), Gandalf strove with the Dark Lord in thought. This is why the Sauron had trouble perceiving Frodo when he put the ring on at the top of Amon Hen. Had Gandalf (the White) not wrestled with Sauron in thought, Frodo's position would soon have been discovered.