Bishop Gossman was ordained a priest of the Archdiocese of Baltimore in Rome, December 17, 1955. The following year he began graduate studies at the Catholic University of America in Washington, D.C., earning a doctorate in canon law in June 1959.
He served as an assistant pastor at Baltimore’s Basilica of the National Shrine of the Assumption of the Blessed Virgin Mary until July 1968, at which time he was appointed administrator of the Cathedral of Mary, Our Queen in Baltimore. He also served as vice-chancellor of the archdiocese from 1959-1968. He was elevated to the position of Monsignor on June 27 1965.
In July 1968, he was named auxiliary bishop of Baltimore and was ordained a bishop on September 11 1968 by Cardinal Lawrence Sheehan. In mid-1970, Bishop Gossman was named urban vicar, with the inner city of Baltimore and its people his special area of ministry.