The fight was stopped in the eighth by the ringside doctor after an accidental head butt caused bleeding on De Leon's forehead. By state law, the fight went to the judges' scorecards and all three had Gamboa ahead, 80-72 by two judges and 79-73 by another.
De Leon bled badly on the forehead after the clash of heads and the fight was stopped at the 1:24 mark of the eighth round.
"He stood with his head up and got cut," Gamboa said.
De Leon agreed with the stoppage.
"I saw it coming but I couldn't get out of the way," De Leon said of the clash of heads. "It was a good stoppage because it was in a spot that couldn't stop bleeding."
Gamboa (21-0, 16 KOs), a Cuban defector, was fighting for the first time since being stripped of his title for failing to make weight prior to his title bout against Jorge Solis last March.
Although he would have liked to win by knockout, Gamboa was pleased with his performance.
"I'm showing it could have been a knockout," he said. "I had to show people what I could do."
De Leon (41-4, 34 KOs), the former WBO junior flyweight champ, suffered his second straight defeat.
"I felt in the early rounds I was winning," De Leon said. "I felt it was getting closer in the end."
The fighters combined for 50 knockouts in 64 fights entering the bout.
Gamboa, 29, won the gold medal in the 2004 Olympics for Cuba, and now lives in Miami.
He appeared to have the upper hand through six rounds in the back-and-forth fight that saw both boxers land hard shots. The faster, stronger Gamboa connected on several hard shots early on, including straight right hands in the second and fourth rounds that landed cleanly.
De Leon battled back, never appearing to be hurt by Gamboa's shots, and landed several hard shots of his own, including a right hand in the fifth round that was his best of the fight.
"I didn't feel he was that strong of a puncher," De Leon said.
Gamboa appeared to take control of the match in the seventh round, seemingly imposing his will on De Leon.
"I saw he didn't have anything from me," Gamboa said. "I'm showing I don't have to be crazy reckless."