Gāndhārī was a north-western prakrit spoken in Gāndhāra. Like all prakrits, it is thus descended from either Vedic Sanskrit or a closely related prior language. Gāndhārī has been found written in the Kharoṣṭhī script. Scholars believe that the language featured elements from the languages native to the area (pre-Indo-European population), as well as Dardic and East-Iranian ethnic languages, which are related to the Indo-Aryan family to which all prakrits belong.
Further Reading
Yu. V. Gankovsky, The Peoples of Pakistan: An Ethnic History. Translated from the Russian by Igor Gavrilov (Lahore: Peoples' Publishing House, 1964)
Reference
Rahman, Dr. Tariq, Peoples and Languages in Pre-Islamic Indus Valley. Available online (http://asnic.utexas.edu/asnic/subject/peoplesandlanguages.html) via The University of Texas.
A characteristic of the Gandharan region was the use of the Gandharilanguage and Kharoshthi script.
Gandhari is a Prakritlanguage derived from Sanskrit and the Kharoshthi script is derived from Aramaic used by the Iranian Achaemenids who conquered the region in the sixth century BC
Before discovery of the manuscripts the knowledge of the Gandharilanguage and Kharoshthi script came from coin legends and inscriptions and from documents from the southern Silk Route in western China where it was used as the official administrative language from the second to third centuries AD.