A recurring source of confusion about Sikh scripture is that there is more than one book, and only one of them is the Guru.
The Guru Granth Sahib is the volume compiled by Guru Arjan in 1604 and completed in the recension attributed to Guru Gobind Singh, and it alone holds the standing of Guru. A second collection, associated with the tenth Guru and compiled after his death, contains a large body of composition of a very different character, including narrative and martial material, and its authorship, its composition and its standing are the subject of a long and continuing argument within the tradition. It is not the Guru Granth Sahib, it is not installed in its place, and the atlas holds it as a separate scripture.
There are further bodies of writing that are revered without being scripture: the compositions of the Gurus' contemporaries, the early accounts of the Gurus' lives, and the code of conduct that regularised Sikh observance in the twentieth century. Each has a standing, and none of them is the volume.
The distinction is not pedantry. Statements about what Sikh scripture says are frequently made on the basis of material that is not in the Guru Granth Sahib at all, and the difference between the volume and the surrounding literature is exactly the difference between what a community treats as its Guru and what it treats as its heritage.