The discounts brought the prison term down to a level where it could be substituted for a final sentence of nine months' home detention.
But the victim's father, who cannot be named to protect the identify of the boy, said he had known Gray for many years and disputed that he had a hard childhood.
He believed he had "weaselled his way out of prison".
"I was a bit disappointed with the outcome of the sentencing," the man told the Herald.
The man has not had contact with the victim for some years due to a protection order, but said he regularly speaks to the boy's maternal grandparents, who now care for him.
They send him photos of the boy, who appears to have recovered from the 15 months of abuse he suffered over the ages of 3 and 4.
"To find out they were being abused ... is atrocious," he said.
"I found out basically the day after Oranga Tamariki went around and took the kids, and my son had black eyes ... it was devastating."
He recognised he might himself receive criticism for not being in his children's lives, but said he was a "different person now" and was working to reach a point where he would be allowed to see the kids again.
"It makes me feel so sad that I wasn't able to be there to protect them, you know. I could have tried harder [when the protection order was served].
"It keeps me up at night."
He believed the sentence Gray received was not enough.
"How can you do that to a child? It just makes me angry because I feel like there's no justice.
"If you can't go to prison for that, what do you go to prison for?"
A petition on Change.org has been started, calling for Gray to be imprisoned for the offending. By Wednesday afternoon it had received at least 2300 signatures.
The offending
According to the summary of facts, the victim was three years old when the abuse against him began in June 2017.
On one occasion in June 2018, Gray became angry with the boy and grabbed his leg, twisting it violently enough to cause a spiral fracture in the femur.
A spiral fracture is when a long bone of the body is torn into two distinct pieces by a rotational force. The boy had to have two surgeries to insert and modify pins in his leg, and had to use a wheelchair while he recovered.
The summary also described Gray "harshly" disciplining the child, and kicking him in the head on at least one occasion. It also revealed the boy was not regularly fed from the time of the leg-breaking incident onwards.
"The victim ... became very intimidated of the defendant Gray and would cower when he entered the room," the summary said.
The victim wasn't taken back to daycare until the end of August 2018, and staff noticed bruising on his face and significant weight loss.
About a week after that, police and Oranga Tamariki went to the boy's home and uplifted him and other children.
He had two black eyes, extensive bruising over his back, thighs, hips and bottom, and some bruising on his arms and stomach.
The child also appeared underweight.
Gray's co-defendant's charges are still before the court.
