Goulart gave his side the lead in the 27th minute from the penalty spot and added his second on 57 minutes.
From the start Guangzhou took the match to their opponents but early in the game had to substitute injured China defender Zhang Linpeng, who was replaced by Rong Hao.
The sides battles each other for dominance of the midfield and in the 25th minute Guangzhou were awarded a penalty when Huang's shot from was adjudged to have been blocked by the arm of Seongnam's Kwak Hae-seong inside the area.
Goulart stepped up and blasted home the penalty to score his seventh goal of the tournament.
Seongnam captain Kim Do-heon had a great chance to score in the 39th minute when he found space inside the area and turned to fire a shot that was well saved by a diving Zeng Cheng in goal.
The visitors came out looking more positive in the second half and nearly scored when Zeng somehow kept out Kim Do-heon's dangerous in swinging corner.
Kim's influence continued to grow and he scored in the 53rd minute but the goal was disallowed and adjudged to be off-side by the linesman.
In Guangzhou's very next attack they scored to make the score 2-0 when Zheng Long's corner was flicked on at the near post by the head of Goulart and the ball went into the far corner of the net.
Seongnam pushed forward with greater urgency in the final 10 minutes and Lim Chai-min had a golden chance deep into stoppage time.
His low shot from inside the six-yard area was blocked by the outstretched leg of Zeng for Guangzhou booked their place in the quarter-final draw on June 18.