The answer to this question can be found in Chapter 6 of The Giver. Specifically, it is found on p. 42 of the paperback edition. The basic answer is that Jonas’s father had to make a special plea in order for Gabriel to get a second year of nurturing.
In the community in which Jonas and his family live, the government controls everything. Children are not born to families in the normal way. Instead, they are born to “Birthmothers” who specialize in bearing children. They are then cared for in a nursery and finally assigned to a family.
The problem with Gabriel is that he is not growing up well enough. Therefore, Father had to make a special plea for him to prevent him from being released. As we see on p. 42,
Father had gone before the committee with a plea on behalf of Gabriel, who had not yet gained the weight appropriate to his days of life nor begun to sleep soundly enough at night to be placed with his family unit.
So, in order for Gabriel to get another year of nurturing, Father had to make a special plea for him. This helps to set up one of the most important parts of the book, in which Jonas leaves the community with Gabriel to prevent Gabriel from being released.