The 2008 Asian champions' new Brazilian coach got off to the worst possible start admitting their heavy home defeat was a 'shame'
The Japanese club, who finished third in the J-League in 2011, were humbled after goals from Tae-Su Kim, Zoran Rendulic and Derek Asamoah.
Serrao replaced long-serving boss Akira Nishino in the close season, with his predecessor overseeing a period of success for Gamba which included the 2005 J-League title, 2007 J-League Cup, 2008 and 2009 Emperor's Cups and victory in the 2008 AFC Champions League.
The Brazilian is acting as a 'placeholder' boss while Wagner Lopes gains his coaching licence to take the reigns at Gamba.
However, Serrao told reporters after his first competitive game in charge that the result was a "shame" and apologised to the club's fans.
"First of all, I'd like to apologise to our supporters. We couldn't play according to the reputation of Gamba. It's such a shame," the Brazilian said.
"We knew our opponents were a difficult one to play against as they have good experience and would come out with counter-attacks.
"At 0-0, we had a couple of good chances but couldn't produce anything out of that while Pohang did on their chances.
"If we had won this opening game today, we could have made a good start and would have taken the upper hand to get through the group stage.
"This is a big loss for us and this means that we have to win away to qualify for the next stage."
Pohang boss Hwang Sun-Hong was delighted with his side's victory against a side he labelled "wonderful".
"We played a wonderful game against a wonderful side," he said.
"Our players did according to what we had planned for today. But we still had to move to achieve our objective."