United substitute Wayne Rooney scored twice moments after coming on, getting his first goal seconds after Gamba striker Masato Yamazaki had pegged the score back to 2-1 in the 74th minute.
Darren Fletcher also got on the scoresheet after coming off the bench as the game exploded into life after Nemanja Vidic and Cristiano Ronaldo headers had given United a 2-0 halftime lead.
"The fans got their money's worth," United manager Alex Ferguson told reporters. "I don't know what the score could have been. Edwin van der Sar's made three tremendous saves."
"They've got no chance. There's no agreement whatsoever between the clubs," Ferguson said.
Yasuhito Endo pulled back a goal from the penalty spot for J-League side Gamba before Hideo Hashimoto smashed his side's third, the sixth goal in 17 crazy second-half minutes.
Rooney's second followed Fletcher's neat near-post header when he latched on to an excellent through ball from Ryan Giggs to sweep home five minutes later.
"It was brilliant out there -- a great atmosphere," said Ronaldo. "We didn't play that well in the second half but we'll be better in the final. We'll be ready for that."
Ferguson's side won the tournament's forerunner, a one-off match between the champions of Europe and South America, with a 1-0 win over Brazil's Palmeiras in 1999.
"We gave them two easy goals from corners and that killed us," he said. "We weren't given a prayer to win this game but we scored three goals so we have to be proud."
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