דברי הימים א, פרק ה׳, פסוק י״א

The settlement of the tribe of Gad on the eastern side of the Jordan River was defined by their close proximity to neighboring tribes and their expansion into specific territories. They established their communities directly alongside and parallel to the tribe of Reuben [רש״י, מצודת ציון, ביאור שטיינזלץ]. Specifically, Gad positioned themselves just north of Reuben's border [ביאור שטיינזלץ], though they eventually expanded eastward well beyond their original boundaries [מלבי״ם].

Their presence in the region of Bashan presents a historical difficulty. Earlier records explicitly state that Moses gave the entire territory of Bashan to the half-tribe of Manasseh. To resolve how Gad could also inhabit this area, one approach explains that Gad did not occupy the whole region, but only a specific portion reaching up to Salcah [רד״ק]. When earlier texts mention Manasseh receiving the entirety of Bashan, they are referring strictly to the lands formerly controlled by King Og and the Argov district. However, there were other sections of Bashan previously ruled by the Amorites, and these specific areas were granted to the tribe of Gad [רד״ק].

Alternatively, another perspective suggests a different arrangement. The land in Bashan stretching up to Salcah was indeed part of Manasseh's original inheritance, but it was subsequently handed over to the tribe of Gad for them to settle [מלבי״ם].