Usami opened the scoring just after the hour mark before Koki Yonekura added a second for the visitors.
Usami added a third in the 86th minute before Yun Ju-tae netting late for the home side.
Gamba started well and dominated possession but did not have any clear-cut chances.
FC Seoul created the best chance of the first-half as Cha Du-ri ran down the right and pulled the ball back for Koh Myong-jin only for him to shoot wide of Masaaki Higashiguchi's post.
Early in the second half FC Seoul's Park Yong-woo had a chance but fired wide.
Two minutes after the hour mark Hiroki Fujiharu's cross was volleyed home spectacularly by Usami into the top right corner giving the keeper no chance.
And ten minutes later Gamba added a second goal after a cross by Fujiharu was floated over to the far post where Yonekura was on hand to send home a low volley past Yu Sang-hun to make the score 2-0.
With five minutes remaining Usami picked up the ball inside the FC Seoul half, ran forward and hammered a low shot into the bottom corner for his second goal of the match.
Yun reduced the deficit for the home side in stoppage time following good work from Kim Hyun-sung to make the score 3-1.
The second leg takes place at '70 Stadium in Osaka on May 27.