The reigning Asian champions went down 2-1 to FC Seoul but had first place in their group wrapped well before the encounter.
The highlight of the night for Gamba was the debut of Usami who scored in his first professional appearance.
"It is a shame about the result. We should have got a win," said Gamba coach Akira Nishino.
"The players that played showed a good approach and worked hard but they need to learn how to develop the game after the scores were level.
"He (Usami) was getting cramp in both legs, but he was still able to play well and scored the goal and I think that indicates he is different.
"This is the time he can be developed. I'd like to use him when the situation is available."