What will happen to Jonas and Gabe?
This sort of depends on whether you're an optimist or a pessimist.
To me, Jonas and Gabe are taking a leap of faith -- they are jumping out of the nest, they are going to have to sink or swim (nice bunch of cliches and mixed metaphors there). They are leaving a world where everything is decided for them and they are going out to see what will happen to them in the unknown world.
In a way, you could say it's like becoming an adult. What happens to you when you leave the protection (but also confinement) of being a kid/teen and become an adult? It can be good or bad, right?
So I hope that Jonas and Gabe will find a way to be happy in their new world, but it's not guaranteed. They have to learn to adjust to a new reality in which they have to make all sorts of decisions for themselves. That can be rewarding, but also dangerous.
Ah - hah! For that, you will have to read Lowry's Gathering Blue, which is not exactly a sequel to The Giver but mentions both Jonas and Gabe in a different community. Gabe is growing up and Jonas is their leader. The interesting thing about this village is that strangers (at least at first) are welcome and people with a handicap or "difference" find their rightful place.
In a third book, The Messager, the open-mindedness of the village changes, but one protagonist named Matt "fixes" things in the end.
In the book "The Giver" Jonas and Gabriel escape to the other, someplace else. Some people have said that they die and the boy Jonas does not really ever develop new memories. I happen to think that he has established his own new memory when he hears music. I hope that he and Gabriel will venture into a community where someone will take them in and raise them in the new world. However, even adjusting to the new environment would be difficult for Jonas. Gabriel, being such a young age, would fare better than Jonas.
They Maked To The Elsewhere. (This Is The Answer.