I keep wondering this. He nearly came to the point of attacking Gandalf with Sting. What if he point blank refused to part with the Ring? What was Gandalf meaning by "You shall see Gandalf the Grey uncloaked!" and how was that a threat? I don't see how Gandalf, even in Valinor Mair-form, could have taken the Ring from Bilbo by force without being corrupted by it in the process.