The Bluewings failed to score in the first leg against Al Sadd last week and proceeded to lose 2-0 but they netted after just nine minutes against Gwangju.
Stevica Ristic, who will miss the game in Doha after being sent off for fighting, shot home from a Lim Kyong-hyun cross.
Suwon stay in third, ahead of FC Seoul on goal difference.
"This was an important game for us," said Coach Yoon Sung-hyo. "We brought in a few reserve players like Lim Kyong-hun and though they haven't played for a while they performed above expectation."
Seoul came back from a goal down to defeat Seongnam Ilhwa Chunma 3-1.
Jung Sung-chan fired Seongnam ahead after 25 minutes with a rocket into the top corner from 25 yards. Ten minutes later Kim tae-hwan's low shot from a similar distance levelled the scores up for Seoul.
With 14 minutes remaining, Dejan Damjanovic's volley on the turn from the edge of the area put the champions in front and the win was sealed with a late header from Mauricio Molina.
In the day's other fixture, bottom club Gangwon FC defeated Daegu FC 1-0 thanks to a goal from Kim Jin-yong.