كتب : Omar AbdelAziz
Gamba won 2-0 in Adelaide on Wednesday, courtesy of a brace from Brazilian striker Lucas Severino.
The Japanese outfit will represent Asia in next month's FIFA Club World Cup on home soil.
Adelaide also make the competition as the highest-placed non-Japanese team in the Champions League.
The Australians meet Oceania representative Waitakere United of New Zealand in a playoff match, as the winner will face Gamba in the Club World Cup quarter-finals.
Egyptian champions Ahli will clash with Coton Sport of Cameroon next week in the CAF Champions League final, after which the winner will make it to the same competition.