She died two days later.
Gardner was convicted of his wife's murder in 2006 and was sentenced to death, which was carried out on January 15, 2020 in Texas.
The 64-year-old received a lethal injection at the state penitentiary in Huntsville as his victim's family watched on through a window.
As he was strapped to the death chamber gurney, Gardner turned to Tammy's son, daughter and mother and said he hoped "one day" they could forgive him.
"I would like to say sorry for your grief," he said.
"I hope what I'm doing today will give you peace, joy, closure, whatever it takes to forgive. I am sorry. I know you cannot forgive me, but I hope one day you will."
After telling several friends watching through an adjacent window that he loved them, he apologised again, adding that he didn't want to talk a lot and would ask the warden to go forward with his punishment.
Gardner's final words were: "I want to see the Lord Jesus so bad. I hope you all understand".
Shortly after the lethal dose of the powerful sedative pentobarbital began, he took three deep breaths and then began snoring. Within seconds, all movement stopped.
He was pronounced dead at 6.36pm local time, 16 minutes after the injection was administered.
Gardner spent 13 years trying to appeal his death sentence, claiming his legal team hadn't done enough to defend him at trial.
Lawyers for the 64-year-old filed an appeal before the US Supreme Court in September, arguing his legal team at the time failed to present evidence he suffered from "abandonment rage".
This is a condition Gardner claimed caused men to kill their female companions with excessive force when faced with recent or imminent abandonment.
The appeal was refused by the court on Monday with Seth Kretzer, one of Gardner's lawyers, saying he was "deeply saddened" by the decision.
"For those of us who are foot soldiers in the long war for conviction integrity, the struggle continues, the hope endures and the spirit never dies," Kretzer said.
HISTORY OF VIOLENCE
Gardner had a long history of violence against his wives, with Tammy being the second partner he had killed.
He shot his second wife while she was pregnant, leaving the 18-year-old paralysed.
She later died from her injuries and her unborn child with her.
He was convicted for aggravated assault over the killing and served an eight-year prison sentence.
While he was still in jail he met his third wife, a woman named Margaret Westmoreland.
Gardner moved in with her and her children when he was released but things quickly turned when he reportedly became violent and threatening to the family.
His actions escalated when he abducted Ms Westmore at knifepoint and savagely beat her daughter.
This landed him back in prison but didn't stop him from threatening to hunt Ms Westmore down if she ever left him.
But by the time he was released from prison he had moved on and promptly married another woman.
They divorced not long after, with the woman applying for a restraining order and claiming Gardner had threatened her with violence and blackmail.
Records show Gardner also sexually assaulted one of his stepdaughters, according to Colin County D.A.
By 1999 he had remarried again and began living with Tammy in Texas, news.com.au reports.
During his trial, prosecutors described their relationship as a "relatively short, but violent marriage" in which he had "dominated, threatened and physically abused" her.
Friends and family testified that Tammy lived in constant fear of her husband, who would hit, choke and threaten her with a gun.
In December 2004 Tammy borrowed money from her company to file for a divorce, resulting in Gardner moving back to Mississippi.
At work Tammy had marked February 7, 2005 on her calendar as the date her divorce would be finalised.
She would go over to the calendar and say, "You're almost there. You're almost there," according to trial testimony.
"She was on pins and needles … hypervigilant, looking out for him," said Curtis Howard, who was one of the prosecutors for the Collin County District Attorney's Office.
But sometime between 11pm and midnight on January 23, 2005, just 15 days before the divorce became final, Gardner broke into Tammy's Texas home and shot her in the head as she was sick in bed.
Despite being shot and unable to hear, Tammy Gardner called 911 two minutes before midnight and told a dispatcher her husband had shot her and he had left in a white pick-up truck with Mississippi plates.
When paramedics arrived they found Tammy spitting up blood with a major head wound.
She was taken to hospital where she fell into a coma and died two days later.
Gardner's sister convinced him to turn himself into the local police.
He was the first person to be executed in the United States this year.
