Only Reuven and Gad asked to settle on the other side of the Yarden. Half of Menashe was also given land to live there.
That is because Moshe was worried that if Reuven and Gad would be isolated from the rest of the Jewish people they might completely disconnect and become their own nation. He therefore forced half of Menashe to settle with these two tribes because the entire tribe of Menashe would continue to keep in contact with each other, and in this way, create a “bridge” between the two sides of the Yarden.
Reuven and Gad’s real intention for settling in the Yarden was to protect Moshe’s tomb, which they knew would be in that area. This is especially true of the tribe of Gad, who were strong and mighty warriors and could best protect it. The reason Moshe’s tomb needed protection was because his physical strength did not fade even after his death. This could have become a source of jealousy and idolatry by the nations of the world.
The reason Gad and Reuven did not disclose to Moshe their real intention was because at that time Moshe was still praying and begging Hashem to let him enter the Holy Land. Therefore, they did not want to insult him or make him feel bad. However, as soon as Moshe realized he would not be entering the Land, he blessed the tribe of Gad and prayed that Hashem bless those who settle where the “legislator” (Moshe) is buried.