"Our confident expectation is that the remainder will be achieved through fundraising," he said.
"I am personally delighted our council has reached this decision. It sends a strong message that we are invested in growth and development for Whanganui."
The decision to put up the $3.9m guarantee was made behind closed doors, though councillor Philippa Baker-Hogan called a vote to bring the discussion into the public part of the meeting.
"These are very important decisions we are about to make. There is no going back on these resolutions and they are a public interest," she said.
"Really the resolution and what is contained in them is already in the public domain... which is reason enough."
But she found no support.
Mr McDouall said he had received legal advice that the debate should be held in confidence.
"There are other details to do with private donors' own personal commitments. That's the real reason," he said.
"We need to respect the privacy of the people who donated significant time and money and not wanting to receive any publicity for it.
"I can commit to everyone here and to the public out there that we will release to the public everything we can. It's only appropriate."
Sarjeant Gallery Trust chairwoman Nicola Williams welcomed the council decision.
"Whanganui ratepayers can be absolutely assured that fundraising efforts will be continuing as we seek support from corporates and small business, individuals, philanthropists and charitable trusts around the country who share our passion for the gallery and the Sarjeant collection."
Mr McDouall said the national significance of the gallery meant about $20m of the money pledged to the gallery had come from outside the region.
"The redevelopment of the Sarjeant Gallery has the potential to bring millions of dollars into our local economy."
