The Gadites and Reubenites, who have large herds of cattle, have requested for their inheritance the land on this side of the Jordan---the area known as the Transjordan. They've agreed to help the other tribes of Israel conquer the land of Canaan and will forfeit their own rights to any territories in Canaan. Moses warned them if they fail to keep their promise to aid Israel in battle they will be sinning against the Lord and will face the consequences.
"The Gadites and Reubenites said to Moses, 'We your servants will do as our lord commands. Our children and wives, our flocks and herds will remain here in the cities of Gilead. But your servants, every man who is armed for battle, will cross over to fight before the Lord, just as our lord says.' Then Moses gave orders about them to Eleazar the priest and Joshua son of Nun and to the family heads of the Israelite tribes. He said to them, 'If the Gadites and Reubenites, every man armed for battle, cross over the Jordan with you before the Lord, then when the land is subdued before you, you must give them the land of Gilead as their possession. But if they do not cross over with you armed, they must accept their possession with you in Canaan.'" (Numbers 32:25-30) Moses knows he will die before Israel enters the promised land and this is why he passes along these instructions to Joshua his successor and to the high priest Eleazar and to the elders of Israel. Moses will not be here when these things come to pass.
In the presence of Joshua, Eleazar, and the elders, the Gadites and Reubenites repeat their vow to assist the other tribes of Israel in gaining hold of the promised land. "The Gadites and Reubenites answered, 'Your servants will do what the Lord has said. We will cross over before the Lord into Canaan armed, but the property we inherit will be on this side of the Jordan.'" (Numbers 32:31-32)
The descendants of Joseph's son Manasseh also desire to live in the land of Gilead and they are counted along with the Gadites and Reubenites when Moses confirms they will inherit this territory. "Then Moses gave to the Gadites, the Reubenites and the half-tribe of Manasseh son of Joseph the kingdom of Sihon king of the Amorites and the kingdom of Og king of Bashan---the whole land with its cities and the territory around them." (Numbers 32:33) Manasseh is called a "half-tribe" because Joseph's family line is reckoned through both his sons: Manasseh and Ephraim. There is no tribe called by the name of Joseph because Joseph's father Jacob, before he died, adopted Manasseh and Ephraim as his own. Joseph had become an Egyptian, for all intents and purposes other than his religion, and he would die in Egypt. Jacob adopted his two sons in his place.
You'll recall that in Numbers 21 the Israelites conquered the kingdom of Sihon and the kingdom of Og when these two kings not only refused to allow the Israelites to pass peacefully through but also went on the offensive against Israel. The cities and towns of these kingdoms will belong to the Gadites and Reubenites who will make improvements to them and fortify them against enemy attacks. "The Gadites built up Dibon, Ataroth, Aroer, Atroth Shophan, Jazer, Jogbehah, Beth Nimrah and Beth Haran as fortified cities, and built pens for their flocks. And the Reubenites rebuilt Heshbon, Elealah and Kiriathaim, as well as Nebo and Baal Meon (these names were changed) and Sibmah. The descendants of Makir son of Manasseh went to Gilead, captured it and drove out the Amorites who were there. So Moses gave Gilead to the Makirites, the descendants of Manasseh, and they settled there. Jair, a descendant of Manasseh, captured their settlements and called them Havvoth Jair. And Nobah captured Kenath and its surrounding settlements and called it Nobah after himself." (Numbers 32:34-42) The descendants of Manasseh will also reside on this side of the Jordan, having conquered Gilead.
Some of these cities were renamed after they were conquered. Nobah renamed Kenath after himself after he conquered it, perhaps to make his ownership clear or to brag about his military prowess. In some instances the cities were renamed because their original names were idolatrous, such as in the cases of Nebo and Baal Meon. By removing the names of false gods from these cities, these names need not be mentioned by God's people. This helps to prevent any legitimacy from being assigned to gods who do not exist. Some of the Israelites will still fall into idolatry over time, but it won't be because they were forced to utter the names of pagan deities when entering these cities.