Who (or what) is Gargan Mish?

Could you mean Gilgamesh?

In the Epic of Gilgamesh, Gilgamesh is credited with the building of the legendary walls of Uruk. An alternative version has Gilgamesh telling Urshanabi, the ferryman, that the city's walls were built by the Seven Sages. In historical times, Sargon of Akkad claimed to have destroyed these walls to prove his military power..

Fragments of an epic text found in Me-Turan (modern Tell Haddad) relate that at the end of his life Gilgamesh was buried under the waters of a river. The people of Uruk diverted the flow of the Euphrates passing Uruk for the purpose of burying the dead king within the river bed. In April 2003, a German expedition claimed to have discovered his last resting place.

It is generally accepted that Gilgamesh was a historical figure, since inscriptions have been found which confirm the historical existence of other figures associated with him: such as the kings Enmebaragesi and Aga of Kish. If Gilgamesh was a historical king, he probably reigned in about the 26th century BC. Some of the earliest Sumerian texts spell his name as Bilgames. Initial difficulties in reading cuneiform resulted in Gilgamesh making his re-entrance into world culture in 1891 as "Izdubar".

Choghā Mīsh (Iranian; ČOḠĀ MĪŠ) dating back to 6800 BC, is the site of a Chalcolithic settlement in Western Iran, located in the Khuzistan Province (Susiana Plain). It was occupied at the beginning of 6800 BC and continuously from the Neolithic up to the Proto-Literate period.

Chogha Mish was a regional center during the late Uruk period of Mesopotamia and is important today for information about the development of writing. At Chogha Mish, evidence begins with an accounting system using clay tokens, over time changing to clay tablets with marks, finally to cuneiform writing system

Do you know what language it is?

Otherwise it is too difficult a question. I can't think of anything in English that explains it.