Despite Gamba taking a 1-0 lead into the second leg in Suita, Hiroshima managed to overcome that deficit and record a 2-1 win to advance on away goals.
Hisato Sato opened the scoring in the 26th minute for Hiroshima before he supplied for Koji Morisaki to add a second before Brazilian striker Dodo pulled one back for Gamba with eight minutes remaining.
Last year's runners-up Kawasaki will meet Jubilo in the semis with Hiroshima taking on Shimizu.
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