Fuelled by methamphetamine, the pair unleashed a slew of gunfire in a "cruel and callous" attack that left the victim unrecognisable.
"Mr Lomax was shot once in the knee, which caused him to be incapacitated, before likely being shot in the chest," Justice Cameron Mander said.
"Neither wound would have killed him instantly."
While he was lying on the ground incapacitated he was shot in face twice from close range, he said.
"During this time, he was also wounded in the head as a result of a succession of shots fired from the .22 firearm."
Lomax's body was left at the riverbed, his face effectively "obliterated and unrecognisable".
The 24-year-old's death caused incalculable hurt to his family, Justice Mander said.
"His family must live with the loss of their loved one forever," he said.
"The nature and circumstances of his death have understandably been a particularly difficult burden for them to bear."
Both Martin and Gosset led criminal lifestyles that were dominated by methamphetamine use, the court heard.
Gosset had already been imprisoned for attempted murder in November 2009.
He had shot someone three times in the chest who was described as lucky to be alive, Justice Mander said.
While Martin had previous convictions that included possession of offensive weapons and firearms, dealing in drugs, violence, and threatening behaviour.
Today Justice Mander jailed both men for life, setting a minimum non-parole term of 17 years and six months for Gosset, and 16 years and six months for Martin.
