Jonas has to leave the community in order to save Gabriel from Release. His main goal is to make sure that Gabriel is not killed; aside from that, there is the secondary goal that once he is gone, the Community will get his memories back, and they will begin to move forward into a new era, away from Sameness.
Jonas's plan for the escape is slightly less specific. He wants to make it to Elsewhere, though he isn't sure how long it will take to get there or what they will do when they arrive. In the last pages of the book, though, we see a glimpse of his hope for his and Gabe's future: "Suddenly he was aware with certainty and joy that below, ahead, they were waiting for him; and that they were waiting, too, for the baby" (p. 178-9). Through Jonas's hope, we can hope for him.