The ending to The Giver is sort of a "take it how you like it" deal. Either Jonas and Gabriel make it to Elsewhere, everyone is happy, and the world is right as rain, or… they die of exposure/starvation in the freezing snow.
Hmm. Not a lot of gray area there.
The first option makes a good case for the book being optimistic. Jonas, despite being raised in the highly controlled community, has come to value freedom and choice over contentment and ease. Gabriel, being a baby and all, could represent the opportunity for a better future. That they make it to their destination means there is hope for all of us.
The second option is a bit more flexible. Clearly, it could be a huge downer. The human spirit, the desire for individuality, etc., are nothing in the face of the rigid rules of Jonas's community. Once we've abandoned freedom for security, we can't ever go back.
Well, that's no good. Maybe you want to go with that other ending…
But wait. Even if Jonas and Gabriel do die, the ending might still be optimistic. The point isn't that Jonas succeeds, but that he tries at all. If he is willing to die for freedom and choice and individuality, then isn't that a kind of victory? He still escaped from the community. He still got "released," in one way or another. And, by leaving the community, Jonas released all the memories he had received from The Giver.
So it may be one small step for Jonas (and one baby step for Gabriel), but it's one giant pain in the butt for the Elders. And that's something.