Garshuni is the writing of Arabic in Syriac characters, and it is one of the clearest signs of what happened to the Syriac churches after Arabic became the spoken language around them. A community whose liturgy and learning were in Syriac letters went on using those letters for a language they were not designed for, adding marks where Arabic had sounds the alphabet could not write. Gamal with its own added mark is the standard answer for the Arabic jim.
The practice ran for centuries and produced a large body of manuscripts: letters, chronicles, theology, medicine and scripture, in Arabic, in Syriac letters, often written by scribes for whom both were ordinary. Catalogues do not always distinguish them clearly, and a text can be shelved by its script rather than by its language.
Three things follow for anyone counting Syriac letters. First, a marked gamal in a Garshuni manuscript is a Garshuni convention and not a letter of the Syriac alphabet. Second, the same is true of the marked teth used for another Arabic sound, and of the marked letters that Sogdian and Persian writers added when they took the Syriac script east. Third, the alphabet itself did not change: twenty-two letters, in an order inherited from Aramaic, with the numeral values that order fixes.
The habit of extending rather than replacing is what let one alphabet serve so many languages without ever being renumbered.