Bhai Gurdas Ji was the scribe of the Adi Granth, writing it out in 1604 to Guru Arjan Dev Ji's dictation. That alone would place him.
He also wrote the forty Vars and the Kabitt Savaiye, in Punjabi and Braj, which are the earliest sustained interpretation of Sikh teaching by anyone other than a Guru. Guru Arjan Dev Ji is reported to have called them the key to the Guru Granth Sahib Ji. They are the only non-scriptural text permitted to be recited in a gurdwara.
His first Var contains the earliest account of Guru Nanak Dev Ji's life and travels that we have, written within living memory of people who had known him.
He was a maternal uncle of Guru Arjan Dev Ji and served as a missionary in Agra and Benares before returning. He supervised the building of the Akal Takht.
He wrote in the plainest Punjabi of anyone in that century and much of it reads as instruction rather than devotion.
The vars are in Punjabi and the Kabitt Savaiye are in Braj, and together they run to a large body of verse. There is also at least one var that the tradition does not accept as his, and the question of what belongs in the forty is a small version of the argument that surrounds the Dasam Granth.
He worked on the compilation and not only as a copyist. The order of the ragas, the arrangement by author inside each one, and the numbering that lets any line be located exactly are all in the volume from 1604, and the man who wrote it out was in the room while that structure was decided.
He is called Bhai and not Baba or Bhagat, and the title carries the point. He is a Sikh writing about Sikhi from inside it with no claim to authority beyond having been there, which is why his verse may be recited in a gurdwara when nothing else outside the scripture may.
Varan Bhai Gurdas.