He was vicar of Beveren-Waas and lecturer of moral theology at the Instruction Centre of Barellieri Ecclesiastical ad Aalst. On 3 July 1959 he was appointed as lecturer of the Major Seminary of Gent.
On 8 August 1959 he was appointed as notary for the diocesan ecclesiastical tribunal, and on 12 July 1961 he became defender of the bond.
He was both honorary canon (29 September 1961) and titular canon (22 September 1967), and also served as parish priest in Landskouter and as a professor at the Seminary.
On 1 September 1970 he was appointed as vice-officialis and on 5 November 1984 as the head of the tribunal. On 1 February 1994 he was appointed judge of the interdiocesan court while continuing as parish priest in Landskouter.
Preconized Cardinal, was elected to the titular Church of Ieper with personal title of Archbishop on 7 October and consecrated on 11 October.
Created and proclaimed Cardinal by John Paul II in the Consistory of 21 October 2003, of the Deaconry of St. Peter Damian ai Monti of St. Paul.
Cardinal Gustaaf Joos died on 2 November 2004.