What did the family have to do in order for Gabriel to be allowed a second year of Nurturing?

Gabriel was not developing in what the elders considered a "normal" fashion. He was underweight but is given an extra year to live with Jonas's family. This was not a common thing to happen. Each family member had to sign an agreement stating that they would not become attached to the baby should things not work out. It is impossible to legislate affection and love; this is one of the major themes of the story.