Having seen Suwon Bluewings go provisionally top with victory over Gangwon FC on Saturday, Jeonbuk moved clear on goal difference after a thrilling seven-goal second half.
Dong-Gook Lee gave Jeonbuk the half-time edge with a goal immediately before the break, but it was in the second 45 minutes that the goals began to flow.
Oliveira Eninho doubled the advantage from the penalty spot in the 61st minute, but Sung-Bin An almost immediately replied.
Lee then scored twice inside five minutes, the first a penalty, to complete his hat-trick with 10 minutes to go.
However, Gyeongnam responded with two goals in four minutes from Jae-Sung Ko.
That set up a nervy finish but Sang-Min Seo's last-minute strike for the hosts settled it.
FC Seoul began the weekend top of the table but they are now one point behind the leading pair after being held 1-1 by 1 Ulsan Hyundai.
Mauricio Molina had put Seoul in front six minutes before the break, but Luis Maranhao levelled almost immediately after half-time and that is how it stayed.
Ulsan are now fifth, one point above Busan I'park, who surrendered an early lead to lose 2-1 at Daegu FC.
Dong-Min Yoon gave Ulsan the lead after just three minutes, but goals midway through either half from Jin-Ho Lee turned the game around.