From this passage and from the subsequent account of the Manassites 1-Chronicles 5:23-24, the Gadites extended themselves to the north at the expense of their brethren, gradually occupying a considerable portion of the tract originally allotted to the "half tribe."

And the children of Gad dwelt over against them,.... Or by them, the Reubenites; and one part of Gilead was given them between them, and the other to the half tribe of Manasseh:
in the land of Bashan, unto Salcah; for though all Bashan is said to be given to the half tribe of Manasseh, Deuteronomy 3:13 yet that is to be understood of the greater part of it; all of that which belonged to Og, but what did not, the Gadites, either from the first, or in later times, inhabited even as far as Salcah, which was one of the cities of Og, Deuteronomy 3:10 and which Benjamin of Tudela (w) makes mention of, being called by the same name in his days.
(w) Itinerar. p. 57.

THE LINE OF GAD. (1Ch. 5:11-26)
the children of Gad dwelt over against them--The genealogy of the Gadites and the half-tribe of Manasseh (1-Chronicles 5:24) is given along with that of the Reubenites, as these three were associated in a separate colony.

The families of the tribe of Gad, and their dwelling-places. - 1-Chronicles 5:11. In connection with the preceding statement as to the dwelling-places of the Reubenites, the enumeration of the families of Gad begins with a statement as to their dwelling-places: "Over against them (the Reubenites) dwelt the Gadites in Bashan unto Salcah." Bashan is used here in its wider signification of the dominion of King Og, which embraced the northern half of Gilead, i.e., the part of that district which lay on the north side of the Jabbok, and the whole district of Bashan; cf. on Deuteronomy 3:10. Salcah formed the boundary towards the east, and is now Szalchad, about six hours eastward from Bosra (see on Deuteronomy 3:10).