Brown, who is a United Nations Special Envoy for Global Education, said this on Thursday in reaction to the release of 21 one of the abducted girls.
He said although the release of the 21 girls "is a huge relief", the development leads to questions about the other girls.
He also expressed concern about how the girls would be reintegrated into the society.
A statement by the former UK PM, who has campaigned for the release of the girls, read, "The release of 21 kidnapped Chibok girls is a huge relief but still raises the question: Where are the other 200 girls and what has happened to them?
"This Friday marks exactly two and a half years since the girls were abducted from their school dormitory. Most were studying to go on to college or university and to be nurses, doctors, care workers, teachers, lecturers and engineers.
"For those girls rescued today – a rescue for which we should thank the International Red Cross – we do not know how they will readjust but one thing is for certain, their lives have changed forever.
"Now we must urge authorities and negotiators not to give up until every girl is safely back with their families."