Why might the Giver already know about Gabriel before Jonas mentions him?
Jonas first tells The Giver about Gabriel in chapter 13. When he mentions Gabriel, who he at first calls his brother, The Giver responds by saying that he knows about Gabriel.
Since he has been The Receiver of Memory, and still is while Jonas is in training, The Giver has to receive all the memories in the community. This obviously means that he has to keep up to date with everything that is happening. One way that he does this is through his video screen. He can request to see whatever is happening whenever he needs to. It is an essential and continuous task. This is one way in which The Giver knows about Gabriel.
Furthermore, as an important member of the Committee of Elders, he may also have heard Jonas's father's request that the newchild should be given an additional year of nurturing. The Committee responded positively to Jonas's father's appeal and declared Gabriel Uncertain.
The Giver has access to way more information than regular people in the community do. I can think of at least a couple of different ways that he might have known:
First, the elders sometimes confer with the Receiver about difficult issues. He might have heard about Gabe in that way.
Second, the Receiver would have been paying a lot of attention to Jonas. After all, he is considering Jonas to be his replacement and so he would surely have wanted to know all about him. This seems to me to be the most likely thing -- he would have found out about Gabe while researching Jonas.
On of the things about Gabriel has been that he is having difficulty sleeping. The giver may have been addressed about the problem with Gabriel in advance of his growing to stay with Jonas' family. Gabriel also has light eyes like Jonas. The giver is able to see colors and may have been aware of the eye color and differences in Gabriel.
I am not sure how much Jonas told the giver about Gabriel until the end of the story, but if Jonas had been sharing information about Gabriel, the giver would know that Jonas has become attached to the child. Because of Jonas' attachment and his awareness of what release means, the giver may have some idea that Jonas will not be able to manage if Gabriel is released.