“and in the valley.” This refers to the east side of the Jordan Valley. For some reason, the tribe of Gad had a strip of land that ran from the main portion of their tribal area east of the Jordan river all along the Jordan River Valley all the way north to the Sea of Galilee. In contrast, the half-tribe of Manasseh did not have any land along the Jordan River. There is no explanation given for why the land was divided like this.
“Beth-haram, Beth-nimrah.” Beth-haram and Beth-nimrah are some five or a little more miles north of the Dead Sea. This means that the territory Reuben had in the Rift Valley was quite small. Eventually, Gad intermixed and even took over some of the towns originally allotted to Reuben on the Plateau.
“Sea of Chinneroth.” This is better known as the Sea of Galilee. The word “Chinnereth” means “harp,” and the Sea of Galilee is shaped like a harp, hence the name. The Sea of Galilee is small, actually, it is a lake, and the entire lake can be seen from the cliffs on both the east and west sides of the lake, so the shape can be easily seen (see commentary on Deut. 3:17).