The feature is optional, and you can undo auto-corrected text if you move quickly.

It could be a while before you see this. The initial rollout focuses on G Suite's Rapid Release domains, who'll see the writing tool arrive within the next two weeks. It'll start rolling out to standard (Scheduled Release) G Suite users on September 12th. As for personal use? Google hasn't said yet -- it might be a while before they're available for everyday use.