Ganapati Bhatt of Karnataka was a staunch devotee of Lord Ganesh. He read Brahma Puran and could know that Brahma in real form was residing at Nilagiri Sri Kshetra Puri. He decided to come Puri and to have Darshan of Brahma. He reached Puri on the day of Snana Purnima, the full moon day of Jyestha of Odiya calendar. Ganapati Bhatta saw Lord Jagannath, Paramabrahma on Snana Mandap with other deities. He keenly observed at Jagannath but he could not observe his dearest Ganesh. Being disappointed he left the temple to go back to his native place. Lord Jagannath could know the feelings of his devotee. He himself as Brahman met Ganapati Bhatt and implored him to come and see Lord once again on Snana Mandap
before leaving Puri. It was evening and on the persuasion of Brahman, he came to Snana Mandap. To his surprise, he saw Lord Jagannath in the head of elephant. He danced on the road saying that God had fulfilled his desire. From that day three deities appear in Gajanan Vesha after taking bath on Snana Mandap.
Source- Odisha Review