Gillen was once commander of the IRA's Belfast Brigade and later became a member of the organisation's Army Council.
He received a substantial amount of compensation for ill-treatment suffered while in detention.
He was arrested in 1987 and interrogated over the murder of a police officer.
He was physically assaulted and suffered a punctured eardrum. He alleged he had been threatened through his interrogation and told that he should become an informer.
A court found that he had been assaulted and he was subsequently released. He was a client of Pat Finucane before the solicitor's murder. He alleged to the Lawyers Committee for Human Rights that RUC officers had made threats to Mr Finucane when representing him in 1988.
Gillen claimed the officers said: "It would be better if he was dead rather than defending the likes of you."
Following the murder of Gerard 'Jock' Davison, Gillen was one of the first senior republicans to arrive in the Markets, along with Bobby Storey and Eddie Copeland, to offer support to the Davison family.