Leo Jan van Gelder was the son of Erich van Gelder and Marie Polak (http://www.joodsmonument.nl/person/537001)
From Genalis:
Source: Civil register - Death
Archive location: Drents Archief
General
No. of finding aid: 0167.030
Item number: 1951
Municipality: Westerbork
Type of record: overlijden
Record number: 28
Registration date: 22-06-1951
Deceased: Leo Jan van Gelder
Gender: M
Date of death: 03-09-1943
Place of death: Auschwitz (Polen)
Father: Erich van Gelder
Mother: Marie Polak
Partner
Additional info.: elders overleden; akte kamp Westerbork; geboortepl: Westerbork; geboortedatum: 02-07-1943