Gerrit Koster was a son of Jesaijas Moses Koster and Rosette Bos. He married in Amsterdam on 18 February 1920 to Betje Walvis, daughter of Zacharias Walvis and Fijtje Posener. website www.wiewaswie.nl.
In addition, a Jokos file (number 25059) on this family is at the Amsterdam Municipal Archive. Access is subject to authorization from the Stichting Joods Maatschappelijk Werk.