What do you mean by this?
Do you mean where the Guru Granth Sahib came from? Who wrote it? The Guru Granth Sahib's authenticity?
I'll hopefully answer your question wih these quotes:
[INDENT]“Vaheguru gave me His Order to sing His Praises day and night. Vaheguru, my Lord and Master summoned me, His minstrel, to the True Mansion of His Presence. (Vaheguru,) the Image of True Praise and Glory, gave me the Siropaao, robe of honour. The spiritual-life giving Name, Amrit Naam, the True Name, which gives eternal spiritual life, has become my sustenance.”
(Guru Granth Sahib, Ang 150)
"O Lalo! As the Lord’s word comes to me so I deliver it."
(Guru Granth Sahib, Ang 722)
"I myself know not what to say; all I speak is what the Lord commands."
(Guru Granth Sahib, Ang 763) [/INDENT]
Thus, we believe that the Gurus had revelations from God, therefore the Guru Granth Sahib is derived from God.
With regards to the authenticity of the Guru Granth Sahib, Macauliffe, an eminent historian, puts it very well:
"The Sikh religion differs as regards the authenticity of its dogmas from most other great theological systems. Many of the great teachers the world has known, have not left a line of their own composition, and we only know what they taught through tradition or second-hand information. If Pythagoras wrote any of tenets, his writings have not descended to us. We know the teachings of Socrates only through the writings of Plato and Xenophon. Buddha has left no written memorials of his teaching. Kungfu-tze, known to Europeans as Confucious, left no documents in which he detailed the principles of his moral and social systems. The Founder of Christianity did not reduce his doctrines to writing, and for them we are obliged to trust to the Gospels according to Matthew, Mark. Luke, and John. The Arabian Prophet did not himself reduce to writing the chapters of the Quran. They were written or compiled by his adherents and followers. But the compositions of the Sikh Gurus are preserved and we know first hand what they taught. They employed the vehicle of verse, which is generally unalterable by copyist, and we even become in time familiar with their different styles. No spurious compositions or extraneous dogmas, can therefore be represented as theirs."
(An excerpt from "The Sikh Religion," by Max Arthur Maxauliffe)
The Guru Granth Sahib was written by the Gurus themselves, thus our scripture has been preserved. We are unique in this respect.