The 19-year-old Usami, called up for Japan's senior squad for the first time earlier this month, is expected to make the switch permanent if his six-month loan deal is successful
"I was worried about leaving the team during the [Japanese] season but chances like this don't come along every day," Usami told a news conference.
"I decided it was the right thing to do to take on this big challenge. To be honest it still hasn't sunk in.
"It's such a surprise to imagine playing in such a big league with such great players," added Usami, a superb dribbler regarded as one of Japan's best prospects.
Usami promised to take his game to another level at the German giants, even warning he would make Dutchman Robben and France international Franck Ribery fight for their places.
"I want to improve my skills to the point I can take their places from them," said Usami, who is poised to leave Japan next month after Gamba's J-League game with Vissel Kobe.
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