I found the following interesting line while reading The Gospel of Sri Ramakrishna by Swami Nikhilananda
Sri Ramakrishna accepted the divinity of Buddha and used to point out the similarity of his teachings to those of the Upanishads. He also showed great respect for the Tirthankarās, who founded Jainism, and for the ten Gurus of Sikhism. But he did not speak of them as Divine Incarnations. He was heard to say that the Gurus of Sikhism were the reincarnations of King Janaka of ancient India. He kept in his room at Dakshineśwar a small statue of Tirthankara Mahāvira and a picture of Christ, before which incense was burnt morning and evening.
[Attitude toward Different Religions,...,The Gospel of Sri Ramakrishna]
The question is about the bolded statement that says the relationship between the Sikh gurus and king Janaka. I want to ask two related sub-questions as a part of this question
Who is king Janaka mentioned here? Is he the father of Sita in Valmiki Ramayana or the liberated Janaka present in several Vedantic scriptures including Tripura Rahasya? Note that I am not sure whether both are the same or different.
Are there any other references saying either explicitly or implicitly, either directly or indirectly that Sikh gurus are possible reincarnations of Janaka?