विस्तारः (द्रष्टुं नोद्यम्)
Prof. Maheswari Prasad (BHU) already speculated this linguistic shift from ‘garta’ -> ‘jatt’ in his paper. I managed to find an actual reference in a Sanskrit text. - PC
e.g. Greek Genos, sanskrit Jana. This phenomena is called the Law of Palatals. The change form K and g to C and J can also be seen in such Sanskrit words as Cakara, Jagama.
One may therefore presume two formations garta and Jarta existing side by side. Whereas garta is available in the Rigveda and later Sanskrit works Jarta existed in dialect. One should not think that the emergence of dialects is a late phenomenon. The philologists opine that already in the Indo-European period the parent language was divided in dialects.
The Trigartas were an ancient people closely associated with the Sibis, Yaudheyas and other groups and belonged to the Ayudhajivi Samgha at the time of Panini. Their location in Hissar as suggested by the Virat Parva is a pointer to the fact that they were the part of the people among whom the Yaudheya clan had excelled.
When the political centres are destroyed by the powerful adversaries, new leadership springs up from the masses and with them come up new names for leadership. In such a situation, the word jarta, an oblique form of gart, appears to have come in prominence. From Jarta to Jatta and then, under the Law of Moves, Jāt is a normal linguistic formation.