Guru Granth Sahib ji

the simple answer, the greatest brahm gyani is our guru.

you dont need to be in search of any gyanis you only need your guru noone inbertween if you are mistaken about this then you are lost and have misunderstood sikhi.

Sikhi Sikhya gurvichar

ther are atleast three great mahapursh that i suggest you read about. they have all left this world but their autobiographies are easily found.

Bhai Sahib Bhai Randhir Singh ji

Sant Baba Mani SIngh uk wale (of akal bunga)

Sant Baba Harnam Singh ji Rampur khere wale