The ending is ambiguous because it is not clear whether Jonas and Gabriel live or not.
Ambiguity means that we are not quite sure what is happening. In the case of Jonas and Gabriel’s escape, we have to base our understanding of what happens to them on what is actually in the text of this book, not the sequels. There are sequels to this book, but the ending of this book was left intentionally vague for a reason.
First of all, when Jonas and Gabriel escape, they are risking their lives. They are being hunted by search planes, they have very little food, and for quite a while they have to be sustained by mostly memories Jonas uses to keep them alive. Soon, they run into terrain that is very rough and harsh. It is cold, and snowy. Jonas can no longer use the bicycle. Things are dire.
It is when things are at their worst, and Jonas has very little strength left, that we start to wonder if they are going to die of exposure. This is when things get interesting. Jonas has previously experienced a memory of snow and a red sled. It was one of the first memories he received. Isn’t it strange, then, that he knows exactly where to find a sled?
Using his final strength, and a special knowledge that was deep inside him, Jonas found the sled that was waiting for them at the top of the hill. Numbly his hands fumbled for the rope. (Ch. 23)
The fact that someone would leave a sled lying around at the top of a hill is odd, as is the fact that Jonas somehow knew how to find it through the memories. That seems to be the implication here, because he says he used his “special knowledge.”
Jonas hears people singing. However, he notes that he may not actually be hearing it. He wonders if it is just another memory.
Behind him, across vast distances of space and time, from the place he had left, he thought he heard music too. But perhaps it was only an echo. (Ch. 23)
One way to interpret these events is that Jonas passed out, and did not find a sled, sled down to safety, hear music and get rescued. Instead, he and Gabriel froze to death. However, as I said, the ending is ambiguous. Since there are sequels, we have to assume that they did indeed survive and someone in that house rescued them.
An ambiguous ending is an amazing thing in literature. It leaves it open to the reader whether to be optimistic or pessimistic about the characters' fates. You also have to go back through the book to look more carefully at clues. Of course, the sequels kind of ruin the effect. Without considering the sequels though, even with foreshadowing, there is enough uncertainty here to make us wonder if Jonas and Gabe lived or died.