The case received national attention as one of the victims, Ashley Ellerin, had plans to go on a date with Kutcher on the night she was murdered. The actor testified during the trial.
Gargiulo maintains his innocence but prosecutors built up an image of him as a killer who presented himself as a friendly neighbour and handyman before breaking into their homes and stabbing them to death.
He was also known as “The Boy Next Door Killer”.
Los Angeles Superior Court Judge Larry P Fidler called his crimes “vicious and frightening,” on Friday at the sentencing as family members of the victims wept.
“Everywhere that Mr. Gargiulo went, death and destruction followed him,” Mr Fidler said at the all-day hearing.
Gargiulo was found guilty of the 2001 murder of Ashley Ellerin, a 22-year-old fashion design student, in her Hollywood home as she prepared to go out with Kutcher.
Testifying at the trial, Kutcher said that he was late to pick up Ms Ellerin, who did not answer her door.
When he looked inside he saw bloodstains that he thought were spilled wine. Ms Ellerin was found with 47 stab wounds.
Her father, Michael Ellerin, who had visited his daughter just hours before she died, spoke at the hearing of his family’s suffering and long wait for justice.
He said he was tempted to imitate his wife Cynthia’s “mournful scream and primal wailing after finding out that Ashley had been murdered”.
He added that her death “marked the beginning of an altered, diminished, heartbreaking life”.
Gargiulo was also convicted of the murder of 32-year-old Maria Bruno, a mother-of-four, in her home in El Monte, east of Los Angeles, in 2005. Ms Bruno’s breasts were cut off and her implants were removed.
He was also found guilty of the attempted murder in 2008 of Michelle Murphy.
Ms Murphy fought him off in her Santa Monica apartment, forcing him to flee and leave a trail of blood that led to his eventual arrests for the other two killings. She was a key witness at the trial.
Ahead of the sentencing, Ms Murphy said in court: “To this day, spending the night alone creates a world of fear in me.”
She cried as she talked about meeting the families of the two women who didn’t survive their attacks.
“How is it fair that one person’s actions can destroy the lives of so many?” she said.
Gargiulo is a former air conditioner and heater repairman, bouncer, and aspiring actor. Speaking before the sentencing, he angrily complained that his lawyers had stopped him from taking the stand in his defence.
“I’m going to death row wrongfully and unjustfully,” said Gargiulo. “I did want to testify and my fundamental choice was blocked.”
Gargiulo is now expected to be extradited to Illinois in connection with the 1993 killing of Tricia Pacaccio in his hometown of Glenview.
In 2011 the Cook County State’s Attorney charged him with first-degree murder of Ms Pacaccio.
Prosecutors in his California trial were allowed to present extensive evidence from that case as they sought to establish a pattern and present Gargiulo as a serial killer. Investigators believe there may be more victims.
California has not executed anyone since 2006 and Democratic Governor Gavin Newsom has halted executions for as long as he is in office.
Courts continue to operate on the assumption that executions will one day resume.
With reporting by the Associated Press