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Sauron was defeated because his major source of power in the One Ring was destroyed. The WK was defeated because his spell of immunity to ordinary weapons was broken. But these circumstances cannot be applied to Gandalf - there is no way he could be defeated unless there was somebody of the required power to break him and his staff (i.e. like how Saruman was stripped of his power and removed from the Order by Gandalf the White).
Who is there that could do that? Gandalf does not depend on a spell to protect him like the WK, nor the Ring as with Sauron. As with Balrogs, only a greater power is assured of victory against the Istari. They are therefore not as vulnerable as the enemy.
As for Gandalf storming the Dark Tower - Eru forbid the Istari to reveal their true power against the enemy, so he could not have done so. Eru intended to get them to reunite the free people of the world against Sauron, rather than ask them to have direct battle with the Darklord (which would take away any point writing LOTR the way it was at all). Even if Gandlaf had demanded a duel, as Sauron belonged to an even higher Mair order, he would most likely break him through possessing the greater power.
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