When Gabe comes to stay with the family for a while, to see if he can mature at an appropriate pace in order to be assigned to a new family, Jonas's family must sign an agreement.  They must pledge that they will not grow too attached to Gabe, and that they will care for him but still willingly give him up for assignment to a "real" family when the time comes.  Even the youngest member of their family had to sign it, because it was against the community's rules for a family to permanently keep more than 2 children.  Keeping Gabe with them was just an attempt at helping him grow to his full ability, not a way for them to adopt a third child.