The hosts had taken the lead in the 64th minute when 17-year-old Takashi Usami scored his first professional goal.

However, Dejan Damjanovic equalised 10 minutes later before Kim struck late on to end Gamba's unbeaten run in the tournament.

The victory means Seoul now need to await the outcome of the game between Sriwijaya and Shandong Luneng later today to know whether they will progress to the last 16.

Gamba were without injured Brazilian forward Leandro while Japan midfielder Yasuhito Endo and Brazilian midfielder Lucas were also out due to suspension.

After securing the top spot in the group two weeks ago, Gamba manager Akira Nishino opted to field a number of youngsters while resting his senior players.

Gamba started well and had their fans on the feet in the 11th minute when teen prodigy Usami smashed a shot goalwards before Masato Yamazaki hit the rebound wide.

The hosts came close again six minutes later when Shuhei Terada pounced on a poor backpass and broke into the area before unleashing a shot that was denied by Seoul goalkeeper Kim Ho-jun.

Seoul produced a good effort in the 30th minute when Brazilian midfielder Adilson hit an effort from edge of the box, forcing Gamba goalkeeper Naoki Matsuyo to punch the ball away. Although Lee got on the end of the rebound, he volleyed the ball over.

Gamba came out reinvigorated after the break and Yamazaki threatened with a shot in the 54th minute before firing wide from Shu Kurata's corner nine minutes later.

But the Asian champions broke the deadlock in the 64th minute when Usami latched on to a through-ball from midfield and bore down on goal before firing home for his first goal on his professional debut.

But Gamba's celebration was short-lived as Seoul responded 10 minutes later.

Serbian forward Damjanovic picked up a cross from substitute Lee Seung-Yeoul and beat Matsuyo to strike home from close range.

Gamba forward Ryuji Bando should have restored the Japanese side's lead in the 83rd minute but his volley was denied by Seoul goalkeeper Kim Ho-jun - and Seoul compounded their opponents' misery in the 92nd minute when Kim slotted the winning goal from close range.