Charles Ellicott Commentary 1 Chronicles 5:9

"and eastward he dwelt even unto the entrance of the wilderness from the river Euphrates, because their cattle were multiplied in the land of Gilead." — 1 Chronicles 5:9 (ASV)

And eastward he inhabited unto the entering in of the wilderness. —As their flocks and herds increased, the Reubenites gradually spread eastward, to the great desert lying between the Euphrates and Syria. This desert was a painful memory to the restored exiles. Ezra took four months to cross it (Ezra 7:9; Ezra 8:22). The form of the expression, unto the entrance into the wilderness from the river Euphrates, seems to indicate that this account was written originally in Babylonia.

Because their cattle were (had) multiplied in the land of Gilead. —Gilead, in Old Testament usage, means all Israelite territory east of the Jordan.