After studies and ordination, he was assigned in Austria as a teacher, where he became noted for his vehement opposition to the Nazi regime; he deemed Nazism as being some warped political tool to create division which was also incompatible with the faith.

His interrogation transcripts reveal how he deemed Nazism and Roman Catholicism to be incompatible, and his strong adherence to his faith prompted Heinrich Himmler and others to refuse the return of his remains for it would result in veneration and silent rebellion.

[5] The beatification process for the late priest culminated after Pope John Paul II beatified him on 24 November 1996 in Saint Peter's Square.

He received a basic education in his hometown and in 1910 entered the high school that the Order of Friars Minor ran at Hall.

[1][5] He served as a soldier on the Italian front from May 1915 until 1916 when he was wounded during a battle; he received the Silver Medal of Courage Second Class as a result of his actions on the battlefield.

[1][3] Gapp later entered the Marianists at Greisinghof on 13 August 1920 for a formation program and began his novitiate on 26 September before being assigned to Graz as a teacher and sacristan from 1921 until 1925.

[1] Gapp's superiors sent him to Tyrol in September 1938 and served as an assistant pastor at Breitenwang-Reutte until October 1938 when the Gestapo ordered him not to teach religion.

He refused to wear a swastika badge and to greet people with the "Heil Hitler" out of conscience, and once in public rebuked a fellow teacher who told students to "hate and kill Czechs and Jews".

He settled for reading British newspapers like The Tablet in order to find uncensored news that would tell him the truth about World War II, and he learned of the Holocaust genocide.

The three went on a picnic close to the border where he found that the two were not Jews but disguised Nazis who abducted him and arrested him in Hendaye on 9 November 1942; he was then sent to Berlin for incarceration.