“Adjoining the territory
From the east side
To the west side,
Gad shall have
Gad was originally on the east side of the Jordan River in Joshua, chapter 13. They were originally surrounded by Manasseh on the north and west side, with Reuben to its south. It was never close to Zebulun. However, both might have been small tribes by this time.
“Moses gave an inheritance also to the tribe of the Gadites, according to their families. Their territory was Jazer, and all the towns of Gilead, and half the land of the Ammonites, to Aroer, which is east of Rabbah, and from Heshbon to Ramath-mizpeh and Betonim, from Mahanaim to the territory of Debir. They got the valley Beth-haram, Beth-nimrah, Succoth, and Zaphon, the rest of the kingdom of King Sihon of Heshbon. This was from the Jordan and its banks as a boundary, to the lower end of the Sea of Chinnereth, eastward beyond the Jordan. This is the inheritance of the Gadites according to their clans, with their towns and villages.”
The tribe of Gad got the northern half of the territory of King Sihon, north of Heshbon, in the center of the captured lands. The Jordan River was their western boundary. They went as far north as the Sea of Chinnereth, which is Galilee. They also got the valleys in between.