The judge said both teens remained in a state of denial but he regarded that as a "coping mechanism".
The violent incident unfolded in an Epsom motel on the morning of December 27, 2014, after the victim had spent several hours driving the teenagers around the city and plying them with booze.
While in the car, the defendants - then 17 years old - formed their plan via text message as they sat in the back seat.
Part of the exchange between the teens included the question: "G should we roll him??"
Crown prosecutor David Johnstone said some of the other text messages during this exchange suggested the severity of the assault that was going to take place, including the question: "Hospital?" And the answer: "Yea G allday (sic)".
A pathologist told the court at trial that Mr Gillman-Harris suffered multiple skull fractures as a result of up to five blows to the head.
He later died in hospital.
Justice Toogood said he was sure the teens had retrieved a bat some time during the night after agreeing one or both of them would provide sexual favours for cash.
The victim then withdrew $400 from an ATM not knowing it was actually a ruse.
The text messages, the judge said, proved it was a "joint enterprise" to beat Mr Gillman-Harris and steal the money.
The victim's youngest brother, Hoani Harris, flew in from New South Wales to be at today's sentencing in the High Court at Auckland and said he struggled to forgive himself.
"I'm sorry, Ihaia. I'm sorry for letting you be alone in hospital. I'm sorry for the pain you suffered on your own. If time could be turned back I'd be in that room with you when you were set upon," he said, turning to the teens in the dock.
"You stole his life. Who gave you the right to steal my brother's life?
"I have no forgiveness for you as human beings."
One of Mr Gillman-Harris' sisters described him as "highly intelligent, knowledgeable and incredibly generous".
"Others may have judged him for his lifestyle but I did not and neither did my other siblings," she said.
Unlike some of the victim's 11 siblings she said she had forgiven the killers and just hoped they got the help they needed.
Nattrass-Bergquist gave evidence that once in the motel room, Mr Gillman-Harris had shown him the beginning of a pornographic video on his laptop and grabbed him.
He said the attack only stopped when Wallace-Loretz returned from the bathroom and struck the victim over the head twice with a bottle but the jury rejected that explanation.
His lawyer, Murray Gibson, said today the defendants had "responded to an inappropriate approach" and would have to carry it on their conscience for the rest of their lives.
Defence counsel for Beauen Wallace-Loretz, John Kovacevich, stressed to Justice Toogood his client's youth and the absence of previous convictions.
Justice Toogood described his upbringing as "appalling" and said the teenager had been living a semi-transient life since leaving school at Year 7.
A report labelled him: "Mostly pleasant but with a deep well of anger."
Ultimately the judge accepted the incident was "at least partly the result of youthful bravado . . . but you failed to appreciate the terrible consequences".
