This link looks incorrect. Can you please provide evidence.
Reginald III of Guelders only had an illegitimate daughter.
He married, in Tervuren on July 1, 1347, Marie of Brabant (1325–1399), lady of Turnhout, daughter of John III, Duke of Brabant and Marie d'Evreux.
They did not have children, but Reginald had an illegitimate daughter, Ponte of Guelders, who married Johan van Groesbeek (d. 1428), lord of Heumen and Malden.
@ John Edward Hersey - I was also skeptical, and I'm supposedly related to Jan Van Hattem, so I did some research to back up whoever made this link and I found a scanned 1886 public almanac (called "Geldersche volks-almanak voor het jaar 1886").
In it, it says (choppy Google translation): Reynald III Duke of Guelders, nicknamed the Fat, who was imprisoned on Nyenbeek let a bastard son after which was called Johan van Hattem, who from 1371-1421 hetslot Hattem possession. In 1422 he sold all his justice at Hattem Gerhard van Cleve. In 1371 he had received money from Duchess Mechtelt Harderwijk "
You can find a link to the scanned copy of the book: http://www.dbnl.org/tekst/_gel004188601_01/downloads.php
It's on page 45-46 of the scanned book.
Apparently Johan van Hattem is also mentioned in "de Graven En Hertogen Van Gelre (1904)" , but I haven't gotten my hands on a copy of that yet.