His research is focussed on the alpha taxonomy of plant-parasitic nematodes, i.e. describing taxa at genus and species level particulary within the Tylenchomorpha. Additionally he is curator of the nomenclatorial nematode database. Gerrit lectures on the taxonomy of plant-parasitic nematodes within the Tylenchomorpha, Dorylaimida and Triplonchida at Wageningen University and at the University of Ghent.
He is also editorial board member of the journals: Journal of Nematode Morphology and Systematics, Russian Journal of Nematology, Nematologia Mediterranea, Redia and Zookeys.