The reason that Guru Nanak Dev Ji gave the honor of Guru to shiva , Brahma , Vishnu and mha Shakti ,may be they have contributed to give spritual knowledge to humans by the source of vedas ,Geeta and many other religious texts and to make people aware that they r gurus not supreme prbrahm (who is not confined to particular god's as He resides in all rich or poor ,human or devtasor demons or other living creat)as they have acted as a teacher to human kind at their time ,do they r called as Guru .As It is the beauty of Shri Guru Granth Sahib Ji that they have given the right place to everyone one they belong to like wise Bhagat Naam Dev ji, Bhagat Kabir Das ji,Baba Fareed ji etc.
