Isauro Aguirre's fate was announced on Wednesday afternoon.
The jury found him guilty of first-degree murder and also guilty in the special circumstance allegation of murder involving the infliction of torture.
This count makes Aguirre eligible for the death penalty, though jurors were explicitly instructed not to consider potential penalties.
The penalty phase, during which jurors will be asked whether Aguirre should be sentenced to death or life in prison without parole, will begin Nov. 27.
Gabriel was routinely beaten, shot with a BB gun, fed cat feces and forced to sleep while gagged and bound inside a small cabinet, witnesses and prosecutors said. He died in May 2013.
After the verdict was announced, Deputy District Attorney Jonathan Hatami was seen crying as he embraced Gabriel's biological father.
In a post-verdict press conference, Hatami said as a father and as a victim of child abuse himself, the case was difficult for him.
"I'm human, and I'm a dad, so yeah, it's hard," he said, as he wiped tears from his eyes. "I come to my job as who I am as a person."
Hatami said he believes that some justice has been served by this verdict.
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"Some closure can be felt by Gabriel's family as far as at least they feel that the system, and I'm part of the system, the system is trying to make some things right and this is a small part in that," he said.
Hatami added that he is involved in child abuse cases because he is passionate about protecting children.
"I believe children need someone to fight for them," he said. "People need to fight for children and others who can't fight for themselves."
Wednesday was the second day of deliberations in the case. Deliberations on Tuesday ended early because two jurors had personal commitments. The seven-woman, five-man panel deliberated for a total of around 5 1/2 hours before reaching its decision.
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During closing arguments, Hatami called Aguirre an "evil" man who "liked torturing" the boy and did so systematically in the months, leading up to the child's death.
Aguirre hated the boy because he thought he was gay, according to the prosecutor, who began his closing argument by displaying a photo of Gabriel's battered body lying on an autopsy table -- covered in injuries head to toe -- as evidence of Aguirre's intent to kill the boy.
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Defense attorney Michael Sklar admitted Aguirre is guilty of beating the boy but argued that the death was accidental. In his closing arguments, Sklar told the jury that Aguirre "acted in a rage of anger followed by an explosion of violence" and not with the deliberation and premeditation required for a verdict of first-degree murder.
Sklar said Aguirre tried to save the little boy and cried after confessing to hitting the boy 20 times.
Gabriel's mother, Pearl Fernandez, is also charged with murder and will be tried separately.
Prosecutors are seeking the death penalty against both Aguirre and Fernandez.
Two former Los Angeles County social workers -- Stefanie Rodriguez and Patricia Clement -- and supervisors Kevin Bom and Gregory Merritt were charged last year with one felony count each of child abuse and falsifying public records in connection with the case.
City News Service contributed to this report.