The jury of seven service members, all men, will resume deliberations Tuesday morning.
Navy prosecutors said the chief petty officer not only killed the young ISIS fighter in Mosul in 2017, but also posed for photos next to his corpse, shot at noncombatants and intimidated other SEALs who might report him.
Gallagher's defense portrayed him as an "old-school, hard-charging warrior" who was targeted by younger "millennial" comrades who harbored "personal animosity" toward him.
The 40-year-old special warfare operator chief faces life in prison if convicted of murder at his court martial at Naval Base San Diego.
The prosecution, the defense and the judge told jurors court Monday that some witnesses had lied on the stand. It will be up to them to decide who they believe.
The prosecution's final say on the case
Commander Jeff Pietrzyk said Gallagher's words, photos and testimony from fellow SEALS proved that he intentionally caused the death of a wounded young ISIS prisoner.
Pietrzyk cited a text message with a photo of the dead man that had the caption "good story behind this, got him with my hunting knife."
"The death of the ISIS detainee is a result of stabbing because Gallagher said so," Pietrzyk said.
SEAL Special Operations Chief Craig Miller testified that he saw Gallagher stab the 12-year-old in the neck and later ask "who's not good with it?"
Former Navy SEAL Dylan Dille told jurors that Gallagher posed for individual and group photos with the prisoner's body.
A few weeks after they returned to the United States, Dille said Gallagher pulled him aside and told him, "I know you guys are talking about what happened. If you stop talking about it, then I'll stop talking about it."
Pietrzyk acknowledged that the victim was not a sympathetic figure. But the victim was in the care of US servicemen and receiving medical aid, Pietrzyk said.
He was "no longer a lawful target," Pietrzyk said.
ISIS might have done worse to its captives, Pietrzyk said, but "we are not ISIS."
The defense argument
The defense did not dispute that Gallagher texted the photo.
Lead attorney Tim Parlatore chalked it up to dark humor and said it did not prove that Gallagher committed murder.
Holes in the investigation, including "no body, no autopsy, no forensics," meant the prosecution had no case, he said.
"We shouldn't even be here, this case should have been dismissed."
Witnesses who testified against Gallagher "put mutiny above the truth," Parlatore said.
Dille also testified that during a sniper mission, he saw Gallagher shoot at civilians, including an elderly man and two women in hijabs.
Dalton Tolbert, another witness, said he fired a warning shot at an old man during a sniper mission with Gallagher. The man ran away, Tolbert said, but fell after another sniper fired.
A third witness said he heard shots fired from Gallagher's position during another sniper mission. Joshua Vriens said he saw four girls who looked to be between the ages of 12 and 14. After the shot, one girl clutched her stomach and fell down.
All three admitted on cross-examination they did not actually see Gallagher fire the shot.
The defense team called eight witnesses, two of whom disputed that Gallagher stabbed the detainee.
One of them, Staff Sgt. Giorgio Kirylo, testified that he did not see stab wounds on the ISIS detainee's neck.
Kirylo testified he moved the body, after medical efforts failed, to take a "trophy photo."
The group wanted to take the photo because "everyone was happy about the day" and the success of the mission, Kirylo said. Kirylo called Miller, who said he witnessed the stabbing, a liar.
Another defense witness, Special Operations Master Chief Brian Alazzawi, said no one ever complained about Gallagher shooting at civilians or stabbing a prisoner while on deployment.
The SEAL who changed his story
Shocking witness testimony came early on in the trial, when a prosecution witness testified that it was he who killed the prisoner, not Gallagher.
Special Operator First Class Corey Scott said that while he'd seen Gallagher stab the prisoner, he was the one who "suffocated" him.
Scott, a SEAL medic, said the prisoner was "unconscious but breathing normally" with a wound on his leg when Scott came upon the scene.
Gallagher and another medic were already there, and someone had performed a tracheotomy -- an incision in the neck to open an airway -- on the prisoner, and a tube was in his neck.
After the other medic left the scene, Scott said he saw "Gallagher pull out his knife and stab the ISIS prisoner underneath the collar bone at least once." He said he did not see any blood. Scott froze and was not sure what to do and said he "stayed with the prisoner until he asphyxiated."
On cross-examination, he told the defense that he never saw any blood after the stabbing. That's when he admitted to killing the prisoner himself, suggesting it was an act of mercy because he was concerned the boy -- a prisoner of the Iraqi forces -- would be tortured by the Iraqis.
Scott, who testified under immunity, did not admit to killing the prisoner in previous interviews with investigators and prosecutors. He said he was only doing so now because he was granted immunity, which means he cannot be prosecuted for his testimony.
He also said he does not want to see Gallagher prosecuted and he does not want to see him go to jail.
In the prosecution's rebuttal closing argument, Lt. Scott McDonald said Gallagher's fate was not up to Scott.
"That's not for him to decide."