He said his father had remonstrated with him, saying if he had spend less time playing sport, he would have done better.
"If we had had this facility, I'm sure I would have done worse."
In September 2013, then an archbishop, Cardinal Dew turned the first sod to mark the construction.
The Catholic Schools Board, based in Wellington, had originally pledged $400,000 toward the gym.
In 2012 Cardinal Dew pledged $1 million, which increased to $1.56 million in 2013. The school raised $230,000 through fundraising drives.
Principal Mr Miles called the gymnasium "a wonderful asset to the school and the wider community. "It's been a long-held vision, a lot of hard work, from people spanning a number of generations. It will significantly enhance the programmes of physical education."
The college had frequently had to take teams off-site because they lacked the room "but that is no longer the case.
"This is a gathering place, a place to build a community."
Cardinal Dew had earlier in the morning held an inaugural mass for the Wairarapa parish at Masterton Town Hall.
