Gamba won 2-0 in Adelaide on Wednesday, two early strikes from Brazilian forward Lucas completing a 5-0 aggregate victory.
The J-League club's reward is a place in next month's FIFA Club World Cup in Japan, involving European champions Manchester United among others.
"I asked the players to kill off the tie in the first half and they carried out the game plan to perfection," Nishino told Thursday's Japanese media.
"We found our rhythm early in the game and decided the outcome quicker than I expected. I couldn't be more delighted."
Gamba's triumph could even prompt renewed calls for the well-respected Nishino to take over as coach of Japan following a string of unconvincing 2010 World Cup qualifying performances.
The Japan Football Association recently gave incumbent Takeshi Okada a vote of confidence after the side's 1-1 home draw with Uzbekistan last month.
Japan midfielder Yasuhito Endo was voted player of the tournament after orchestrating last week's 3-0 home win over Adelaide and the semi-final defeat of holders Urawa Reds.
"Our Japanese rivals Urawa won it last year and I knew it would be Gamba's turn this season," said Endo. "We came in with confidence and the margin of victory demonstrates that."
Gamba captain Satoshi Yamaguchi added: "It's a relief to finally win the title. This was the one we wanted to win since before the season began."
Adelaide had already qualified for the lucrative Club World Cup in Japan as the top non-Japanese side in the Asian Champions League.
"We learned a hard lesson," said Adelaide coach Aurelio Vidmar. "We did exceptionally well to get to the final but Gamba took the game to a different level -- a level we're not used to."
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