In the years 1955-1964 he received his philosophical and theological education in Great Britain and Italy in Comboni seminaries.
After returning to Sudan, Gassis was a parish vicar in Wad Medani in the central part of the country.
The Sudanese government filed an indictment against Gassis after he made a statement to the US Congress in which he informed about crimes committed by the authorities against his own people.
For his activities in defense of the Christian population of his country and calling for respect for human rights by the government in Khartoum, Gassis was awarded many prizes.
He informed the public about the religious persecution of the Christian population, the bombing of human settlements, terrorism, hunger and murders.