Gamboa, fighting for the first time in more than a year, survived a ninth-round knockdown to take a unanimous 12-round decision over Michael Farenas of the Philippines in the 130-pound fight.
It was the first fight for Gamboa (22-0) with 50 Cent, and it went longer than either fighter or promoter would have liked. Gamboa knocked down Farenas in the second round and again in the seventh, but every time he tried to finish him off he ended up getting tagged himself.
Gamboa, who lives in Miami, opened cuts around both eyes of Farenas early and used his quickness to land some big shots. But Farenas (34-4-4) was relentless and nearly turned the fight around when he landed a huge left hook in the ninth round as Gamboa was trying to drop him again.
Gamboa, who won an Olympic gold in 2004 before defecting from Cuba, got up and finished the round but was shaky. He fought cautiously the rest of the way, content to win the decision.
50 Cent, who got his Nevada promoter's license last month, reportedly paid $1 million for the contract to promote Gamboa.
Before the fight, 50 Cent was lowered into the ring from the rafters of the MGM Grand arena as he rapped one of his songs.