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

Jonas's father has gone before the committee of Elders to plead for permission that Gabriel should be allowed an extra year of nurturing since he has not attained the weight appropriate for his days of life and has not yet started sleeping soundly enough to be placed with his family unit. It is normal procedure that such a child is deemed inadequate and then released. 

Gabriel has already been spending time with Jonas's family unit, and it seems as if he will recover because Jonas has found a way to soothe him and make him sleep. The committee labels Gabriel Uncertain and decides to give the child a special reprieve before his Naming and Placing so that Jonas's family unit can assist with his development for an extra year.  

Gabriel will still be nurtured at the Center but would spend his nights with Jonas's family unit. He will, however, only be a temporary guest and each family member, including Lily, has to sign a pledge that they will not get too attached to him. Furthermore, each family member also has to agree that they will freely give him up without protest or appeal when he is assigned to his family unit at the following year's Ceremony.

Jonas is happy with this arrangement for it means that they will still see Gabriel regularly because he would be part of the community. If he should be released, he will be sent Elsewhere never to be seen again.

At this point in the novel, Jonas does not know what being released means. He later, in chapter 19, to his utter shock and dismay, discovers that it means to be killed. He sees images of his father practicing the procedure on a newchild that does not meet the requirements.

When, in chapter 21, his father tells Jonas that Gabriel is to be released, Jonas decides that he is going to rescue him. He and The Giver have already decided that he (Jonas) is going to leave the community. At this point, Jonas decides on a change of plan so that he can take Gabriel with him. In the end, he successfully escapes with the newchild who has become like a brother to him.