Gandalf a "steward?"

[Boromir88]

While reading an excellent post on another forum it posed the question of whether Gandalf saying that he was also a "steward" is more symbolic/metaphorical or if we can see Gandalf as actually being a "steward."

"If you understand it, then be content," returned Denethor. "Pride would be folly that disdained help and counsel at need; but you deal out such gifts according to your own designs. Yet the Lord of Gondor is not to be made the tool of other men's purposes, however worthy. And to him there is no purpose higher in the world as it now stands than the good of Gondor; and the rule of Gondor, is mine and no other man's, unless the king should come again."
"Unless the king should come again?" said Gandalf. "Well, my lord Steward, it is your task to keep some kingdom still against that event, which few now look to see. In that task you shall have all the aid that you are pleased to ask for. But I will say this" the rule of no realm is mine, neither of Gondor, nor any other, great or small. But all worthy things that are in peril as the world now stands, those are my care. And for my part, I shall not wholly fail of my task, though Gondor should perish, if anythig else passes through this night that can still grow fair or bear fruit and flawer again in days to come. For I am a steward. Did you not know?"

Tolkien establishes a difference between Denethor's definition of "Steward", and Gandalf's definition of "steward." Notice the use of capitilized, uncapitilized.

There can be two different meanings behind the word Steward. Denethor takes his "Stewardship" as he is the "Lord of Gondor," he is it's absolute ruler, and he throws this lackluster response at the end..."I remain in power! Ohhh...unless the king returns." Where another possible word for steward is "guardian." They look over, protect, land/property for it's master.

I think we can all see that throughout the book Gandalf fulfills this stewardship role, he is the caretaker of Middle-earth. But is this supposed to be taken more symbolically? Or is Gandalf the actual steward of Middle-earth? The "steward" of Eru? "I am a servant of the Secret Fire, wielder of the flame of Anor." In that matter, could we also think that all the Istari were the "stewards of Middle-earth," just they strayed from their "stewardship" task?