The Giver explains to Jonas that he will gain something by being the receiver of memories. What will he gain?
Compared to what he was like before he became the receiver of memories, Jonas will be gaining a lot. In essence, he'll be gaining his humanity.
Before, when he was just a normal person in the community, Jonas was not really human in a pretty basic way. He had essentially no emotions. Perhaps most importantly, he could not feel love.
By giving him the memories that he did, the Giver allowed Jonas to feel love. This made him much more human than he had previously been.
So Jonas will gain the ability to have real emotions. This will make him more human, and that is what he will gain by becoming the receiver of memories.
The people in the community see everything in the same way. They dress the same. They can not see colors. They do not have any idea about love or sexuality. Their experiences have never included joy and laughter. They have also never included sadness and pain.
The Giver is transferring these memories to Jonas a little at a time. Jonas is gaining experiences and memories in his mind. The new sensations are sometimes good and sometimes bad. By becoming a receiver of the memories Jonas gains the experience of the sensations. He also gains the ability to read and keep books around. In addition he will be allowed privacy.