He was known for his numerous stage and screen roles, including Mr. Dekker in Torchwood: Children of Earth (2009) and Kevan Lannister in Game of Thrones (2011–2012; 2015–2016).

[3] Adopting his mother's maiden name of Gelder for his stage name, he began acting professionally in an ITV Sunday Night Theatre episode in 1972.

He appeared in television programmes such as Torchwood: Children of Earth in 2009, and Game of Thrones, 2011 as Mr Dekker and Kevan Lannister respectively.

In 2019, he guest-starred as Librarian Scholar Charles in the BBC TV series His Dark Materials, based on the critically acclaimed book trilogy by Philip Pullman.

[8] Gelder's partner Daniels stopped working to take care of him, and remarked that he dealt with his "dreadful illness with such bravery".