Gigira took a 1-0 lead before half time when July Elijah scored in the 34th minute to put Gigira in front.
FC Port Moresby responded in the second half through a double from Jamal Seeto to see them steal a 2-1 lead in the 69th minute.
But Gigira leveled up a minute later through Ian Yanum.
The game looked certain to go into a draw with the clock winding down.
But it took brilliance to steal the win during extra time and Gigira ' s Moses Arisim when given the ball didn't let his his team down powering a ball through from outisde the penalty box to beat the goal keeper.