The spiritual light of the ten Guru Sahibs is enshrined within Sri Guru Granth Sahib Ji, and thus the commandments of Gurbani are to be adhered to. From 'Ik Oankaar' to 'Atharah Das Bis'. Gurbani is to be accepted as the Guru.
Raagmala was authored by Guru Ji. First, Sri Guru Arjan Dev Ji had Bhai Gurdas Ji write an edition of Aad Sri Guru Granth Sahib Ji in which Raagmala is present. It is written in the same ink, on the same quality of paper and in the same handwriting as the rest of the Gurbani (this was also confirmed by various Sikh scholars, as recorded in the book Kartarpuri Bir De Darshan). This edition is now at Sri Kartarpur Sahib (Doaba). Bhai Banno Ji copied that edition, which also includes Raagmala. At Takhat Damdama Sahib, Sri Guru Gobind Singh Ji had Bhai Mani Singh Ji as scribe and dictated the whole of Sri Guru Granth Sahib Ji, in which Raagmala is present. It is also present in the four editions written by Shaheed Baba Deep Singh Ji.
Those who argue that the poet Jodh wrote Raagmala in Madavanal Kamkandla, or that the poet Alam wrote it, are mistaken. The poet Jodh wrote Madavanal Kamkandla in Sanskrit in the Hijri year 991, which is 1640 Bikrami, and Raagmala is not present in it. The poet Alam was one of Sri Guru Gobind Singh Ji's 52 poets; he lived from 1712 Bikrami to 1774 Bikrami and wrote his Raagmala according to what he heard spoken in the court of Sri Guru Gobind Singh Ji. Alam lived 113 years after the first edition of Sri Guru Granth Sahib Ji was compiled. How can it be argued that he wrote Raagmala 113 years before the first edition? From this it is clear that Guru Sahib wrote Raagmala.
Bhai Sahib Bhai Vir Singh Ji, in Sri Gur Partap Suraj Granth from page 2128 to page 2133, explains in detail why Raagmala was authored by Guru Ji. If one were to read Giani Sahib Singh Ji's (Damdama Sahib) detailed discourse on the authenticity of Raagmala, no confusion or doubt would be left. There is also a small book called Raagmala Gurbani Hai, published by Damdami Taksal, which details the spiritual meaning of Raagmala and answers every point that has been raised against its authenticity. Furthermore, the book Anbhavi Arth by Sant Sundar Singh Ji Bhindranwale gives the in-depth spiritual meaning of Raagmala; those arths are the basis of the Arths page on this site.