Gurmukhi has thirty-five base letters in its traditional order, which begins ੳ ਅ ੲ ਸ ਹ and then runs through the consonants in five groups of five by place of articulation, followed by ਯ ਰ ਲ ਵ ੜ.
That order is not alphabetical in the European sense; it is phonetic, organised by where in the mouth the sound is made, inherited from the Brahmi tradition. Children learn it as the Painti, the thirty-five.
Six additional letters with a dot below, the pairin bindi letters, were added later to write sounds from Persian and Arabic: ਸ਼ ਖ਼ ਗ਼ ਜ਼ ਫ਼ ਲ਼.
The script was standardised by Guru Angad Dev Ji from the Landa and Sharada traditions already in use among Punjab's traders and clerks. Landa was a merchant shorthand without vowels, effectively unreadable by anyone who did not already know the content, and one of the changes was to write the vowels.
Gurmukhi is written with a headline like Devanagari but is a distinct script with its own letterforms.
It has three tones, which are not marked, and which is why a learner reading Gurmukhi correctly can still be unintelligible.
The thirty-five letters are learned in a fixed order beginning with the three vowel carriers, and the order itself is what a Punjabi child recites first.
Guru Angad Dev Ji standardised the script from the Landa and Sharada family, and the name means from the mouth of the Guru.
It was made for a language that had been written in traders' shorthand without vowels. Fixing the vowels is what made a scripture possible, which is the point of the whole exercise.