The garden, pre-fall imagery
The imagery of the titular Garden becomes a full-blown abstract imagery that shapes the nostalgia and agony of the novel, because the child remembers the community and love that used to fill that garden, and like the fall from Eden, he knows that it can never be regained again on this earth. He visits the desolate garden like a prophet in the wreckage of a desolated city, wondering about the nature of suffering and death. The imagery creates a field of Biblical associations, beginning with the fall of man in the Garden of Eden.
Community and love
Through romantic love for Micol and community love for his friends and their families, the narrator describes the full cost of the Holocaust in his personal life. The Nazis came through and stole his loved ones from him, robbing him of community, joy, and love. He remembers a time of friendly hangouts with his Jewish friends, flirting with the sisters of those families, playing tennis with the families, and loving them deeply. The portrait is not of "six million dead," it is of a few very important Jewish people whose death he mourns specifically and personally.
Numbers and names
The narrator is committed to a personal approach to mourning. He doesn't get consumed by numbers; in fact, when numbers arise in the imagery of the novel, he becomes detached and complacent. Notice, for instance, that he is failing math class, because the numbers seem meaningless to him in light of the real deaths of his friends. The novel implies the dehumanizing act of tattooing Jews in concentration camps with numbers. This dehumanizing association between people and numbers can also happen when a person thinks of the Holocaust for its death tolls without remembering that each person was a person with a name and a community and hopes and dreams.
