In the 1970s Gerrit Knaap (1954) studied history at Utrecht University, specializing in social and economic history, including historical demography. In 1985 he also obtained his Ph.D. at that University with a thesis about the VOC (the United Dutch East India Company) and the population of Amboina in the second part of the seventeenth century (promotor prof. dr Theo van Tijn: copromotor: dr Jurrien van Goor). During the middle of the 1980s he was also active as an editor of source publications, among others in the capacity as a free-lancer for the Royal Institute for the Tropics in Amsterdam. At the end of the 1980s Gerrit Knaap was employed as a post-doctoral researcher in prof. dr Heather Sutherland's project on the VOC harbourmaster's specifications at the Free University in Amsterdam. In 1990 he was employed by the Section Languages and Culture of Southeast Asia of Leiden University to give a course about the Economy of Modern Indonesia.

In the middle of 1990 Gerrit Knaap became head of department Documentation History Indonesia/curator Special Collections at the Royal Institute of Languages and Anthropology (KITLV) in Leiden and at the same time member of its management team. One of his first improvements in the department was the digitalization of the catalogues. A second improvement was the change of the department's name from Documentation History Indonesia to Historical Documentation, because the special collections of the KITLV not only included Indonesia but also the Netherlands Antilles and Suriname. Furthermore, he developed a policy to bring the special collections under the attention of a wider public through exhibitions, side-flanked with book publications. The first result of this was the Woodbury & Page project, which dealt with the most prolific photographers in nineteenth-century Indonesia with a book publication and several exhibitions in the Netherlands and in Indonesia (Rijksmuseum voor Volkenkunde in Leiden, Erasmushius in Jakarta and several other places in Indonesia). Because of the very positive reactions to this project  Gerrit Knaap decided to convert the digital catalogue of the KITLV photographic collection into a pictorial image database accessible on the internet. This was soon followed by another exhibition-cum-publication project on the first Indonesian professional photographer Kassian Cephas, which was launched with an exhibition and a booklaunch in the kraton of Yogyakarta, opened by his Majesty the Sultan of Yogyakarta, Hamengkubuwana X. At about the same time he was one of the persons taking the initiatitive and hence becoming secretary of the Foundation Oral History Indonesia, which established a digital interview archive with Indies and Indonesian migrants coming to the Netherlands in the period between 1940 and 1965.

In 1998 Gerrit Knaap obtained a sabbatical year, partly sponsored by the Netherlands Organization for Scientific Research (NWO), at the Netherlands Institute for Advanced Study (NIAS) in Wassenaar for a follow-up of his post-doctoral project about the VOC's harbourmasters' specification, this time not about Java, but about South Celebes (Makassar). At about the same time the name the departement Historical Documentation was changed into Archives and Images. In 2002 he established in the framework of the 400 year commemoration of the VOC, for a general public the website www.voc-kenniscentrum.nl. 

In 2006 Gerrit Knaap left the KITLV and became programme leader of the theme 'The Dutch and the Culture Overseas' as well as member of the management team of the Netherlands Institute of History (ING) in the Hague. In this capacity he also was project leader of 'The Dutch in the Caribbean World, circa 1670 to circa 1870' and 'Religious Policies Netherlands Indies, 1814-1942'. In 2008 on the invitation of Els Jacobs. he designed the project 'Bookkeeper General Batavia; The Circulation of Commodities of the VOC during the Eighteenth Century'. In 2011 the NWO funded ING merged with the Huygens Institute of the KNAW to become the Huygens Institute for Netherlands History (H-ING), under the umbrella of the KNAW. Besides leader of the just mentioned projects, in 2013 he was on behalf of the KNAW also nominated as a parttime endowed professor of Overseas and Colonial History at Utrecht University. In 2020 he retired from H-ING and continued his activities, mostly at Utrecht University as a professor emiritus.