Wilhelm Friedrich "Gaius" de Gaay Fortman was born in Amsterdam on 8 May 1911 to an orthodox Reformed Protestant family. The De Gaay Fortman family were descendents of 17th century Walloon immigrant Jacques Le Gay, and became one of the foremost Neo-Calvinist families in the Dutch Patriciate, with prominent reverends, scholars, business(wo)men and politicians.
The Reformed De Gaay Fortman was a progressive politician of the Anti Revolutionary Party, the party which later merged with other Christian parties to form the Christian Democratic Appeal politician. He was a Public servant, secretary of the government labour negotiation team and a teacher at the CNV-school (Christian Labour Union). Later he became a professor at the Vrije Universiteit and its Rector Magnificus. In 1956 he was unsuccessful as informateur during the long 1956 cabinet formation. However, he was able in 1960 to quickly resolve a cabinet crisis. In 1973 he, together with Boersma, were persuaded to become a minister in the Cabinet Den Uyl. He had a good relationship with the formerly Reformed social-democrat party leader Joop den Uyl. As Minister of Home Affairs he proposed a plan to divide the Netherlands into 24 mini-provinces and he played a key role in the independence negotiations for Suriname in 1975. In 1981 he again acted in a cabinet formation as informateur and managed to pave the way for a government of CDA, PvdA and D66. De Gaay Fortman was in favor of co-operation of the ARP and later the CDA with the PvdA (left). He refuted offered positions in Christian Democrat – Liberal coalitions. De Gaay Fortman became a member of the CDA, but he became alienated from the party mainstream. In his view the CDA too much emphasised policies that resulted in the dismantling of social security. In the parliamentary elections of 1994 he endorsed Gert Schutte, the leader of the Reformed Political League. He is buried at Zorgvlied cemetery.
His son Bas de Gaay Fortman followed in the political footsteps of his father and became leader of the Political Party of Radicals in the Second Chamber and later a Senator for its successor, the GreenLeft party.