The City Mission provides day services for men, a permanent men's night shelter – with plans for a women's night shelter – and operates from a new $6.4 million facility funded and built debt-free.
Gorman said he was "intensely proud", modestly crediting those around him with the agency's success.
"My staff do an incredible job, in sometimes really difficult situations," he said.
"When I see my staff able to develop people so they can move on with their lives, that's a real highlight."
Change was good for an organisation, Gorman said, "as long as it is purposeful".
"Maybe a younger, fresher person with new ideas will be able to bring new vitality to what is a splendid organisation."
Gorman took up the position in 2004, after 20 years as a social worker with Child, Youth and Family. He was a priest at parishes across Canterbury before entering the social work field.
The Canterbury earthquakes in 2010 and 2011 were "the most challenging time" in his career, but brought out the best in himself, his staff and the community, he said.
"We only lost a week of being closed, and I'm really pleased about that looking back.
"There is no manual written about what you do when a quake comes, so I had to make it up as did a lot of people – it was important to give confidence to staff, to clients and to people, but basically I didn't have a clue."
"If you can make that connection, I think people forgive you the mistakes, as long as you're there for them, and that's what I've tried to do."
There had been "a staggering change" in the people coming to the mission, Gorman. Those who would have once donated now knocked on the door for help.
"The working poor, they are a new phenomena for us – we're living in an economy where things cost quite a bit.
"Rents are very high, for example, but wages are not, so there is a gap there which has created a really big strain on all the agencies in Christchurch."
Gorman had deferred his retirement until January to oversee building plans for a permanent women's night shelter across the road from their Hereford St facility.
He was "excited about what the future might hold", starting by filling his calendar with visits to grandchildren and travel with his wife, Jan.
"I need some time to just think, and get my mind clear again, because it's been a busy 12 years.
"The mission is really well supported, and I'd like to see that continue."
Father Peter Williams, of the Christchurch City Mission, said it had been "a privilege for many of us to work alongside Michael".
"Not only has he managed the mission's growing service of the disadvantaged in the city with competence and care, but he has also been a much respected face and voice for compassion in the community."