The Guru Granth Sahib is written throughout in Gurmukhi, and the script's development is bound up with the volume it carries. Its letters descend from the same family as the other north Indian scripts, and its arrangement and standardisation are associated in Sikh tradition with the second Guru.
The practical problem it solves is specific. The Gurus were composing in a register that drew on several languages, to be sung by people who read none of the prestige scripts of the region. Persian and Arabic were written in a script tied to a learned and largely Muslim readership; Sanskrit and its descendants in a script tied to another. A body of composition addressed to everyone needed an orthography that belonged to none of these and could carry all of them.
Gurmukhi does that, and the volume's spelling records sounds and forms with a consistency that has made it a substantial source for the history of Punjabi. The archaic orthography of the Granth, with its final vowels and its particular conventions, is itself an object of study, and reading the volume accurately requires knowing those conventions rather than applying modern Punjabi spelling to it.
That exactness is why this atlas has not entered the Gurmukhi headings of the rag sections. A plausible spelling would look correct to almost every reader and be wrong, and in the script of a living scripture that is not an acceptable trade.