[the phantom]

how is it that Tolkien has characters, who are not privy to our knowledge courtesy the narration's omniscience, submit to Gandalf?

Perhaps those who are the most noble and good can somehow sense the nobility and goodness of Gandalf?

Faramir is without a doubt the purest, if you get what I mean, of the Steward's household (Denethor, Boromir, Faramir), and notice that he is the one who is accused of being a "wizard's pupil".

Also, what about the hobbits of the Shire? You don't see Ted Sandyman or Lotho Baggins being chummy with Gandalf, do you? No, you see Frodo, Sam, Merry, and Pippin following him- the best of the hobbits.

It seems to me that Gandalf (particularly Gandalf the White) has only a thin veil over his nature, which is undoubtedly "good", which means that his goodness can be sensed by anyone with a halfway perceptive heart and mind. Therefore, those who are both good and perceptive are likely to follow him without much hesitation.

To see how this works, simply look at the way the hobbits react to Aragorn when they first meet him and don't know if they should trust him. Which hobbit is the first to give Aragorn the benefit of the doubt? Frodo- the most perceptive of the hobbits. Frodo senses Aragorn's goodness. Frodo said this to Aragorn-

'I believed that you were a friend before the letter came,' he said, 'or at least I wanted to. You have frightened me several times tonight, but never in the way that servants of the Enemy would, or so I imagine. I think one of his spies would- well, seem fairer and feel fouler, if you understand.'

Eomer also senses Aragorn's nature when he first meets him. Like Frodo, he trusts Aragorn enough to essentially place his life in Aragorn's hands. He even says it-

In this I place myself, and maybe my very life, in the keeping of your good faith. Do not fail.

Eomer also liked Gandalf. When he learned of his death he said this-

'That is heavy tidings,' said Eomer. 'At least to me, and to many; though not to all, as you may find, if you come to the king.'

As you can see, certain characters, such as Eomer and Frodo, just know who to trust. It must be a result of them perceiving the true nature of others.

In Gandalf's case, his true nature is probably less hidden than most. He is the white rider, sent by the Valar and then sent back by Eru himself! He is the very symbol of opposition to Sauron. Because of this, I'm not at all surprised that Sauron's opponents find it easy to take orders from Gandalf.