He was Bishop of the Diocese of Nidaros in the aftermath of the introduction of Lutheranism into Norway.
He also undertook a rescue mission for Nidaros Cathedral which was badly damaged by fires in 1327 and again in 1531.
In his efforts to initiate reform of the church, he found an ally in Gjeble Pederssøn (ca.
Hans Gaas served as superintendent and Bishop in Trondhjems stift from 1549 until his death in 1578, except for the war period from 1564 to 1570.
The war ended in 1570 with the withdrawal of Swedish forces under terms of the Treaty of Stettin at which time Gaas was reinstated in his former office.