I think that part of Sauron's character is that he can't understand why Gandalf views the ring as corrupting. Maybe he should be able to figure this out, but I was describing how Sauron might think, not how he should think.

As for whether Gandalf would get power from the ring — It's true that the ring wouldn't give Gandalf as much power as it would give Sauron. But it would still give Gandalf a lot of power.

In some other essay, Tolkien speculates about who would win in a one-on-one fight between Gandalf-with-the-ring and Sauron-without. Unfortunately, I can't remember what Tolkien said the outcome would be, and I can't find a copy online at this moment[*]. However, I do remember that he said that it would be close, a lot closer than if neither had the ring.

Tolkien also says somewhere (in LOTR or in an essay) that the ring would make someone like Gandalf an effectively unstoppable military leader in some unspecified way (IIRC).

* ETA: Here's at least some excerpts from that "essay" (actually a letter): http://www.americanidea.org/handouts/06240110.htm Tolkien doesn't give a final verdict on who would win. The key passage:

Gandalf [with the ring] might be expected to master him [ie, Sauron without the ring].

[...]

One can imagine the scene in which Gandalf, say, was placed in such a position [of possessing the ring and fighting Sauron one-on-one]. It would be a delicate balance. On one side the true allegiance of the Ring to Sauron; on the other superior strength because Sauron was not actually in possession, and perhaps also because he was weakened by long corruption and expenditure of will in dominating inferiors. If Gandalf proved the victor, the result would have been for Sauron the same as the destruction of the Ring; for him it would have been lestroyed, taken from him for ever. But the Ring and all its works would have endured. It would have been the master in the end.

Gandalf as Ring-Lord would have been far worse than Sauron. He would have remained ‘righteous’, but self-righteous. He would have continued to rule and order things for ‘good’, and the benefit of his subjects according to his wisdom (which was and would have remained great).