The goals were scored by Ricardo Bueno's early spot kick and Hwang Ui-jo's second half goal.
In the 8th minute Brazilian forward Bueno slotted home a penalty, after Hwang was hauled down inside the penalty area by Shohei Ogura, to make the score 1-0.
Gamba did very little in the first-half get back in the game apart from Yasuhito Endo's free-kick that was fired directly at goalkeeper Park Jun-huk.
Seven minutes after the half-time break Seongnam went close to doubling their lead when Kim Do-heon raced into the penalty area and unleashed a furious strike on goal from an angle that Higashiguchi did well to push away around the post.
The hosts did look the more likely to score again and with 22-minutes remaining they did through Hwang as the striker latched on to Kim Tae-youn's flicked header before curling his shot past Higashiguchi to make the final score 2-0.
Seongnam's goalkeeper denied Patric in stoppage time from close-range as the visitors were forced to settle for a second consecutive defeat.