Deniz Hummet scored the winner in the 30th minute to earn the J-League outfit their second continental title, which comes 18 years after they claimed the Asian Champions League.
The result means Ronaldo has yet to win a major trophy since joining Al Nassr in 2023 after his contract with Manchester United was terminated by mutual consent, although his side hold a two-point lead in the SPL with one round of matches remaining.
The Saudi side dominated the clash at the King Saud University Stadium but were unable to find a way past Gamba's young goalkeeper Rui Araki.
The 18-year-old palmed Abdulrahman Ghareeb's seventh-minute shot around the post while Sadio Mane hit the side-netting 16 minutes later as Al Nassr made the early running.
But on the half-hour mark, the Japanese side went in front when Hummet turned from 16 yards out to slot Issam Jebali's through pass into the bottom corner of Bento's goal.
Ghareeb was inches away from the equaliser three minutes before the interval, and Ronaldo glanced a stoppage-time header wide with Jorge Jesus' side on top.
Al Nassr's frustration continued as Joao Felix hit the post in the 77th minute from distance with Araki finally beaten.
Gamba defender Takeru Kishimoto slid in to make a match-saving block of the Portuguese forward's shot six minutes from time after Ronaldo's backheel from the edge of the area momentarily put his teammate into space.
