The main motivation for Jonas to get Gabriel to Elsewhere in The Giver is learning what Release means. Most people in the community have no idea that Release means “kill” the person. Jonas does not find this out until he begins his training with The Giver and asks to see his father release a newchild.
He killed it! My father killed it! Jonas said to himself, stunned at what he was realizing. He continued to stare at the screen numbly. (ch 19, p. 150)
After Jonas realizes that Gabe is in danger of being murdered, he decides he has to rescue him. Jonas finds out Gabe is scheduled to be Released soon after seeing this ceremony.
"So," Father went on, "we obviously had to make the decision. Even I voted for Gabriel's release when we had the meeting this afternoon." (ch 21, pp. 164-164)
This is when Jonas decides to take Gabe and leave earlier than planned.
Another motivation for Jonas’s leaving is to return the memories back to the people and help them feel again. He and The Giver discuss it and decide that the people need and deserve those collective memories. However, his main motivation is to rescue Gabe.
Lowry, Lois (1993-04-26). The Giver (Newbery Medal Book). Houghton Mifflin Harcourt. Kindle Edition.