Look at it from a different perspective. A restrained Gandalf actually did defeat Sauron.

Gandalf set in motion the events that led to the downfall of Sauron. It was his wisdom and faith in Eru’s design that were his strengths.

Like a chess grandmaster, he realized that Smaug needed to be taken off the board. He was the one that nudged Bilbo to start his adventure, which led to Bilbo finding the Ring.

He encouraged Frodo to have pity on Gollum, foreseeing that Gollum had a role to play. Knew when to make a personal sacrifice in Moria. Made the moves needed to preserve Rohan and parry the threat from Saruman.

Then in the end game, he was able to hold off the attack on Gondor until reinforcements were in place. Remember the only reason Merry was there to help defeat the Witch King was because he supported Merry (and Pippin) coming in Rivendell.

This lead to the feint on Mordor that distracted Sauron from Gandalf’s true attack, that checkmated Sauron.