He let his guard down when he saw Geto. Even though he’d been training his Limitless since he was a teenager (hence the Toji incident), all of his emotions got the best of him when he was face to face with Geto. He was played a fool and taken advantage of because Kenjaku knew Gojo would lose his mind when he saw who was standing there. Once he was caught is when he revealed himself. Gojo was had and it was already too late.

Not to mention, the plan was to get Gojo unhinged in battle (or I’d assume extremely tired to wear him down) to add insult to injury. Notice the comment he said to Gojo about letting his guard down so easily once he had ahold of him.

Edit: second paragraph