[Raynor]

I wouldn't think so; Gandalf himself states:

Originally Posted by The White Rider, TTT

I have spoken words of hope. But only of hope. Hope is not victory. War is upon us and all our friends, a war in which only the use of the Ring could give us surety of victory. It fills me with great sorrow and great fear: for much shall be destroyed and all may be lost. I am Gandalf, Gandalf the White, but Black is mightier still.

In the letters, Tolkien also notes:

Originally Posted by Letter #156

[Gandalf the White] is still under the obligation of concealing his power and of teaching rather than forcing or dominating wills, but where the physical powers of the Enemy are too great for the good will of the opposers to be effective he can act in emergency as an 'angel' – no more violently than the release of St Peter from prison.

[He also notes that Gandalf reveals his might when rescuing Faramir and forbiding the entry of the witch-king in Minas Tirith]. As far as invicibility goes, this is not something that was achieved by either Melkor or Sauron - I doubt that Eru would break the rules with Gandalf. We should also consider that in Myths Transformed, HoME X, it is said:

Originally Posted by Notes on motives in the Silmarillion, iii

[Manwe] had commanded Sauron to come before him for judgement, but had left room for repentance and ultimate rehabilitation. Sauron had refused and had fled into hiding. Sauron, however, was a problem that Men had to deal with finally: the first of the many concentrations of Evil into definite power-points that they would have to combat, as it was also the last of those in 'mythological' personalized (but non-human) form.