[The Squatter of Amon Rûdh]

Spot on, Adanedhel. That much is explicitely stated by Gandalf himself.
Petty Dwarf's point about fire revealing the One Ring, and the interesting parallel between this and the Ring of Fire that Gandalf wears, is an interesting one, although possibly coincidental. In the Silmarillion, when Círdan meets Gandalf at the Havens on his arrival in Middle-Earth, he concentrates on Gandalf's rôle as an agitator against Sauron's blandishments, and the requirement that the Istari kindle hearts to courage and resistance, although there may well have been more to it than that: Tolkien was a great one for details of that sort.