Together with his brother Alexander de Gelsey, they played an instrumental role in returning the more than 300-year-old Royal University Catholic High School (which was closed in 1948) to its original building. It took three years of litigation to get the building back from the capital's municipality. The brothers set up the Catholic University High School Foundation, which contributed more than 100 million forints to the modernisation of the building. William de Gelsey personally secured the application for the 'EEA and Norwegian Financial Mechanisms' tender as well as co-financed 15% of the project (€150,000).
