Would he have done it?

[Kuruharan]

To answer the original question...

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...Gandalf would have made good detestable and seem evil.
Ltr. 246

...so I think this clears up any question of any other alternative than the destruction of the Ring being Good. It wouldn't have been. There was no other saving of Middle-earth.

However, I'm afraid Gandalf himself would disagree with those of you who think he would not have taken the Ring in extremities.

"No!" cried Gandalf, springing to his feet. With that power I should have power too great and terrible. And over me the Ring would gain a power still greater and more deadly." His eyes flashed and his face was lit as if by a fire within. 'Do not tempt me! For I do not wish to become like the Dark Lord himself. Yet the way of the Ring to my heart is by pity, pity for weakness and the desire of strength to do good. Do not tempt me! I dare not take it, even to keep it safe, unused. The wish to wield it would be too great for my strength
emphasis mine

There is here a (I think a very healthy) self-skepticism. I think this illustrates that Gandalf was not immune from the call of the Ring and feared he might succumb. In fact, the whole quest could probably be viewed as Gandalf's desperate attempt to avoid taking the Ring himself.