The country allotted to the tribe of Gad appears, speaking roughly, to have lain chiefly about the centre of the land east of Jordan. The sought of that district--from the Arnon (Wady Mojeb), about halfway down the Dead Sea, to Heshbon, nearly due east of Jerusalem--was occupied by Reuben, and at or about Heshbon the possessions of Gad commenced. They embraced half Gilead, (3:12) or half the land of the children of Ammon, (Joshua 13:25) probably the mountainous district which is intersected by the torrent Jabbok, including, as its most northern town, the ancient sanctuary of Mahanaim, and you can find more about that here on st-takla.org on other commentaries and dictionary entries. On the east the furthest landmark given is "Aroer that is before Rabbah," the present Amman. (Joshua 13:25) West was the Jordan. ver. (Joshua 13:27) The character of the tribe is throughout strongly marked--fierce and warlike.
Main reference: Smith's Bible Dictionary (1860s)
Bible Gallery and Bible Illustrations (thousands of images)