He was born and worked in Antwerp. He is sometimes confused with a "Gerard van Duynen/Deynum" who worked in the Hague and who was possibly the father of Isaac van Duynen. This G. van Deynum is known for still life paintings in the manner of Jan Davidszoon de Heem whose works from the period 1640–1650 he copied. He is possibly a descendant of the Guilliam van Deynum (1575–1624) known for portraits in Antwerp. He died after 1654.