Amber DuBois' mother got her wish and met with her daughter's killer, but said she is keeping the details of her daughter's final moments to herself to spare other family members.
Gardner, 31, will be sentenced on Friday. He is expected to get life in prison.
McGonigle said she was determined to talk one-on-one with her daughter's murderer and even confronted Gardner's mother on Tuesday, asking his mom to give up her visiting time so McGonigle could see him instead. Gardner's mother tearfully refused.
McGonigle got a call Wednesday afternoon saying she could meet with Gardner. She rushed to San Diego's Central Jail, according to ABC News' San Diego affiliate KGTV.
McGonigle said Gardner was nervous and respectful during their meeting, but she declined to detail what he said out of respect for Amber's father, Moe DuBois. The father, who is separated from McGonigle, has said he did not want to talk with the killer or hear about his daughter's last moments.
Amber's grandmother, Shelia Welch, said that spoke with her daughter after the prison meeting, but did not want to hear what he had to say, KGTV reported.
McGonigle said she will use what Gardner told her to write the victim's impact statement she will read at Gardner's sentencing on Friday.
The girl's mother told ABC News last week that she wanted to know how Gardner got her while she walked to school and why.
"The last hour of her life was spent with this freak," McGonigle said. "If I get some of my questions answered I'll be a lot better."
"I don't want to know gory details. I don't want to know if she suffered," she said. "I don't want to hear if she cried for me."
Gardner, a convicted sex offender, has said in a jailhouse interview that he carried out the attacks during a "major rage."
"I was aware of what I was doing, but I could not stop myself," he said in a jailhouse interview with San Diego television station KFMB. "I was in a major rage and pissed off -- pissed off at my whole life and everybody that's hurt me and blew up, and I hurt the wrong people."
DuBois vanished Feb. 15, 2009, just a few blocks from her school. King disappeared Feb. 25. Her body was found five days later.
DuBois had been excited that morning about a Future Farmers of America project and was carrying a check for $200 to buy a baby lamb.
Witnesses later said they saw her walking with a tall man. McGonigle thinks that man may have been Gardner. She had just talked with Amber the week before about never going off with a stranger.
"The only way he would have got her ... is if he had a knife or a gun and walked up on her," she said.
Was he in a rage the entire time he drove Amber 30 minutes north of her hometown, where she was killed and buried, McGonigle wondered? What did they talk about? What did she say to him?
Sara Muller Fraunces, a spokeswoman for Chelsea King's parents, declined to comment on whether they'd be interested in ever speaking to Gardner.
"They're keeping their feelings private," she said. "They'll speak at the sentencing and that's where they're focused."