The holy book Guru Granth Sahib contains the writings of 36 saints, four of whom lived in Varanasi - Sant Ramanand, Sant Kabir, Sant Ravidas and Sant Surdas.
Guru Nanak visited Varanasi in 1506 and collected the writings of Kabir, Ravidas and other saints, which were incorporated into the teachings of the Sikh faith.
The Gurubagh Gurudwara was built at the place where Nanak stayed in Varanasi. You can visit it if interested, it is a small gurdwara but important to the Sikhs because it is the location where Nanak had discussions with orthodox pandits explaining the beliefs of Sikhism.
Photos of the gurudwara are here, on the Historical Gurudwaras website.