He arrived at Pinelake Health and Rehab Center moments after the shooting started.
Garner says it didn't take him long to realize something was seriously wrong, so he went into the home alone without backup.
"I was going through the building and I was taking my time and there was a lot of rooms and doors everywhere and I was just, I didn't know where the guy was at," Garner said. "We kind of met up in the hallway together. He had a gun in his hand and I ordered him several times to put the gun down which he lowered in my direction and that's when he fired."
Shotgun pellets hit Garner in the leg. He fired at roughly the same time and hit alleged gunman Robert Stewart in the chest.
Garner says he is thankful more people were not killed and says he wants to be back at work by the beginning of May.
On Tuesday, officials also released autopsy reports from two of the victims. It revealed 78-year-old John Goldston died from a gunshot wound to the abdomen and 88-year-old Jesse Musser died from a gunshot wound to the back.
Investigators have not released the other autopsy reports yet.
On Monday, a grand jury indicted Stewart on eight counts of murder.