In August 1967 Gajek, joined the Congregation of Marian Fathers. Between 1967 and 1974 he studied at the Catholic University of Lublin. In late 1960s father Sergiusz became engaged in activities of the exiled Belarusian clergy. He had contacts with the Belarusian cultural and religious centre in London, including bishop Ceslaus Sipovich and Father Robert Tamushanski. On 23 June 1974 he was ordained priest and served for several years in a parish in Głuchołazy.
The following two years Gajek studied at the Theology Faculty of the Catholic University of Lublin. After that he has been sent to Rome, where in 1978-1983 he studied at the Pontificia Instituto Orientale. In November 1983 Gajek became Doctor and returned to Poland. In 1983-1999 he worked at the Catholic University of Lublin.
As leader of Greek Catholics in Belarus
In 1994 Pope John Paul II has appointed Fr. Sergiusz Gajek Apostolic Visitor for Greek Catholics in Belarus. In 1996 Gajek became Archimandrite and corresponding member of the Pontifical Academy of Mary. In 1997 he became adviser of the Congregation for the Oriental Churches.
Fr. Sergiusz is author of numerous scientific publications on Christianity in the Slavonic East.