If you just go by the information provided in The Giver, there is a lot of ambiguity.
As he approached the summit of the hill at last, something began to
happen. He was not warmer; if anything, he felt more numb and more cold. He was not less exhausted; on the contrary, his steps were leaden, and he could barely move his freezing, tired legs. But he began, suddenly, to feel happy. He began to recall happy times. He remembered his parents and his sister. He remembered his friends, Asher and Fiona. He remembered The Giver.
By the end of the book, Jonas and Gabriel are not in a good place. They're starving, and all the time spent walking in bad, ever-worsening weather has left them in great danger of freezing to death. The above quote is a bit strange, then, in the way it sums up how awful Jonas' physical condition is, yet follows it up with how Jonas suddenly begins to feel happy for no reason and thinks about happy times and good people. You could either interpret this as being Jonas' last effort to push himself forward, his mind casting about for anything that will give him the strength to journey on. However, you could also easily see this as the point at which Jonas loses his grip on reality as his body begins to shut down for good. As he dies, his mind latches on to the things in life he loved most. Whichever way you choose to see it, the text doesn't give any confirmation as to which one is the right interpretation.
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. He settled himself on the sled and hugged Gabe close. The hill was steep but the snow was powdery and soft, and he knew that this time there would be no ice, no fall, no pain. Inside his freezing body, his heart surged with hope.
Here's another weird element. Why would a sled just be sitting on top of a hill, waiting for someone who would be insane enough to make the journey Jonas and Gabriel decide to make? Of course, it is possible that the sled is real. Maybe the people in the happier place Jonas is trying to find put it there because they hope others will be brave enough to try to find them. On the other hand, it seems far too in keeping with the memory Jonas possesses about the sled and the hill. This could just be Jonas' mind playing tricks on him, making him hallucinate in his starved, frozen condition. Once again, the text itself gives us no confirmation as to which is the case.
He forced his eyes open as they went downward, downward, sliding, and all at once he could see lights, and he recognized them now. He knew they were shining through the windows of rooms, that they were the red, blue, and yellow lights that twinkled from trees in places where families created and kept memories, where they celebrated love. Downward, downward, faster and faster. 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. For the first time, he heard something that he knew to be music. He heard people singing. 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.
Now is the time you as a reader must decide whether to be optimistic about Jonas and Gabriel's position. The sled seemed like it could be pulled straight from Jonas' mind, and this parallel between what he is describing and what is already in his mind continues in the passage above. The Christmas trees with their pretty lights, the loving families, and the joyous singing are all things Jonas has experienced while receiving memories from the Giver. Is it a simple coincidence that this real, warm and loving salvation Jonas has found so closely resembles the images he has clung to in his mind? Perhaps Jonas, aware that he is dying and has failed to save Gabriel, is given a merciful last thought of the love and joy he so lacked in his life back at home. And, in the final lines where he believes he hears singing coming from that very home, he could either be hearing the real singing of a society that has finally awoken to joy or be desperately imagining it so that he can be happy in the knowledge that at least that part of the plan worked.
The catch to all of this, of course, is that you can't really know the final fate of Jonas and Gabriel based on the actual text. It's left up to you! Now, if you want outside help in determining what happened to them, I would recommend reading the companion books Gathering Blue and The Messenger, in particular. There are references that provide more information about the ending of The Giver.