Izak van Gelder was a son of Salomon van Gelder and Johanna Feitsma. He married in Amsterdam on 6 February 1918 to Judic Walvis, daughter of Zacharias Walvis and Fijtje Posener. Website www.wiewaswie.nl.
In addition, a Jokos file (number 55602) on this family is at the Amsterdam Municipal Archive. Access is subject to authorization from the Stichting Joods Maatschappelijk Werk.