The murderer, serving 17 years, tackled Khan with a chair and narwhal tusk before helping wrestle him to the ground allowing armed police to shoot him dead outside Fishmongers’ Hall.
Mr Gallant was approved for release from prison earlier this year because of his bravery.
Ignoring advice from a prison officer to stay put as the attack unfolded Mr Gallant followed the officer to find him giving first aid to a woman and another woman lying on the floor.
In his first interview since being released, he told how Khan then came towards him armed with two knives and a fake bomb strapped to his body.
He told Channel 4 News: “He came towards me and he opened his jacket and showed me what was an explosive belt strapped to his waist.
“I think he wanted to scare me off. But he was in the midst of a killing spree. I couldn’t just walk away and for some reason, I assumed it was fake.
“And then I looked next to me, and there was a chap next to me holding out a narwhal tusk.”
Mr Gallant hit Khan with the tusk which snapped but managed to force the terrorist out onto the street where he was ultimately shot dead.
Gallant become friends with Mr Merritt by the Learning Together prison reform programme.
He said: “Jack was amazing – very intelligent, very caring.
“This guy was passionate about his role and he had such a profound effect on so many people in the prison system.”
He additional: “Prison can work. But I think it’s got to come from within.
“If you don’t want to change, nothing can change you.”
London Mayor Sadiq Khan paid tribute to the victims two years on, saying: “Today we honour and remember Saskia Jones and Jack Merritt who were tragically killed in the appalling terrorist attack at Fishmongers’ Hall two years ago.
“Saskia and Jack chose to dedicate themselves to helping others. They were passionate about changing the world for the better, and we continue to draw inspiration and hope from the way they lived their lives.
“I would also like to pay tribute to the heroic efforts of our emergency sets and the bravery of ordinary Londoners who ran towards danger that day to help save the lives of strangers.
“The way our city came together and stood united in the aftermath of the attack showed the world once again that those who seek to divide us and destroy our way of life in London will never succeed.”
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