Would he have done it?

[Boromir88]

I think we are all aware of Letter 246 where Tolkien says if anyone would be expected to beat Sauron, it would be Gandalf. So, there's an established possibility that Gandalf could beat Sauron with the Ring and usurp his place. We also know that the Istari were forbidden to match their power with Sauron. So my simple question is...

If absolutely necessary (and I mean no other possible way of defeating Sauron for good...i.e. destroying the ring) would Gandalf, for the good of Middle-earth, broken the rule set on him and overthrow Sauron? This basically comes down to whether you think Gandalf cared enough about Middle-earth to save it by using the Ring and defeating Sauron, if there was no other way (while risking a lot of horrible consequences). Or whether Gandalf (who was aware of the consequences) believed that eventhough this was Middle-earth's only chance, there would be no victory in the long run.