He was a son of Johannes van Keulen (1654-1715) and Louysje Laurend Coerte. Gerard marries Ludowina Konst. In 1704 their only child was born, John II of Cologne.

On October 7, 1712 Van Keulen was granted a patent for Het gilded licht der Zeevaart. He thus extends the patent law that was started by his father.

Van Keulen acquires through succession of Willem Jansz. Blaeu and his sons for themselves and inherit the privilege of being employed as mapmakers by the Vereenigde Oostindische Compagnie.

He takes over the family business from his father in 1714. His father dies in 1715. At that time the company specialized in cartography, publishing and selling marine atlases, maps and guides, books, producer and distributor of navigation instruments.

Van Keulen ensured variation in the range by switching to the sale of individual large-format cards, which could be bound.

After his death, his son and his widow continued the business.