Most of the thirty-one measures in the Guru Granth Sahib are shared with the wider Indian musical inheritance and would be recognised by a musician trained anywhere in the north. A handful are not, or are used here in forms that the classical systems do not preserve, and Majh is the standard example.
This matters for a practical reason. The performance tradition of the Granth, carried by hereditary musician families and later by trained kirtan singers, is the only place some of these measures survive as living practice. Where a rag has fallen out of the general repertoire, the Sikh tradition holds the form because the scripture requires it, and the requirement is specific: the heading on the page names the measure, and singing the hymn means singing it there.
The consequence for scholarship is that the Granth is a musical document as well as a religious one. Attempts through the twentieth century to reconstruct how the rags were performed in the Gurus' own time have had to work from a tradition that has itself changed, and different lineages of musicians preserve different readings of the same measure. Whether current performance reflects sixteenth and seventeenth century practice is not settled and probably cannot be.
None of that uncertainty touches the text. The headings are fixed, the order is fixed, the pagination is fixed. What varies is the sound, which is the part that was never written down, because notation was not how any of it was transmitted.