Played in Shandong, the visitors had their star playmaker Yasuhito Endo sent off prior to the main break and it seemed as though the perfect start to their title defence may come undone.
But the Japanese outfit held strong and managed to go ahead in the 58th minute through Leandro.
The Brazilian striker collected a pass inside the area and finished coolly past Shandong keeper Li Leilei.
The win has Gamba on top of the Group F table with a maximum 15 points from five outings whilst Shandong remain in second place on seven points and just goal difference ahead of FC Seoul.
Gamba have secured their place in the round of 16 but Shandong will need to defeat Indonesian club Sriwijaya i their next match or hope Gamba defeats Seoul.