After receiving theological education, he was ordained on 1 December 1935 as a Catholic priest of the Byzantine Rite.
On 17 February 1958, Pope Pius XII appointed him exarch and titular Bishop of Gratianopolis.
On 12 March 1958, Gad was ordained bishop by the patriarch of the Melkite Greek Catholic Church Maximos V Hakim, who co-celebrated with the Roman Catholic Archbishop of Athens Marios Makrionitis and Roman Catholic Bishop of Syros and Milos Georgios Xenopoulos.
From 1961 to 1964, Hyakinthos Gad participated in the I, II, III and IV the sessions of the Second Vatican Council.
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