European champions United, heavy favourites to become the first British team to win the seven-team FIFA tournament, will face Ecuador's LDU in Sunday's final in Yokohama.
Victory was overshadowed though by fresh speculation in the Spanish media linking winger Cristiano Ronaldo with a move to Real Madrid, reports angrily denied by manager Alex Ferguson.
"Do you think I would get into a contract with them?", Ferguson told reporters. "They've got no chance. There's no agreement whatsoever between the clubs.
"I wouldn't sell them a virus. It can be an angry situation and annoying but we know their game. The best thing for us is to ignore it."
"The fans got their money's worth," added Ferguson. "I don't know what the score could have been. Edwin van der Sar made three tremendous saves to secure the win for us."
Yasuhito Endo pulled back a goal from the penalty spot for Gamba before Hideo Hashimoto smashed his side's third in injury time, the sixth goal in 17 crazy second-half minutes.
"I don't care, I don't read the newspapers," he shrugged. "We're out here in Japan and I don't know anything about what's happening around me. I'm very happy at Man United."
Ferguson's side won the tournament's forerunner, a one-off match between the champions of Europe and South America, with a 1-0 victory 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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