A passage from Nichiren's On Repaying Debts of Gratitude with commentary

Question: Is there a correct teaching that was not propagated even by T’ien-t’ai and Dengyō?

Answer: Yes, there is.

Question: What sort of teaching is it?

Answer: It consists of three things. It was left behind by the Buddha for the sake of those who live in the Latter Day of the Law. It is the correct teaching that was never propagated by Mahākāshyapa orĀnanda, Ashvaghosha or Nāgārjuna, T’ien-t’ai or Dengyō.

Question: What form does it take?

Answer: First, Japan and all the other countries throughout Jambudvīpa should all make the Shakyamuni Buddha of the essential teaching their object of devotion. In other words, the Shakyamuni and Many Treasures who appear in the treasure tower, all the other Buddhas, and the four bodhisattvas, including Superior Practices, will act as attendants to this Buddha. Second, there is the sanctuary of the essential teaching. Third, in Japan, China, India, and all the other countries of Jambudvīpa, every person, regardless of whether wise or ignorant, will set aside other practices and join in the chanting of Namu-myoho-renge-kyo. This teaching has never been taught before. Here in the entire land of Jambudvīpa, in all the 2,225 years since the passing of the Buddha, not a single person chanted it. Nichiren alone, without sparing his voice, now chants Namu-myoho-renge-kyo, Namu-myoho-renge-kyo."

Commentary:

"First, Japan and all the other countries throughout Jambudvīpa should all make the Shakyamuni Buddha of the essential teaching their object of devotion. In other words, the Shakyamuni and Many Treasures who appear in the treasure tower, all the other Buddhas, and the four bodhisattvas, including Superior Practices, will act as attendants to this Buddha."

Therefore, the Gohonzon is nothing other than Shakyamuni Buddha of the essential teaching.