You need the Gilding the Bilious quest from your GC. (The linked page shows Twin Adder, but they each have a version of that same quest.) It in turn requires running Arum Vale, so I think you also have to have done the Going for Gold quest that unlocks Arum Vale itself. Pick up Going for Gold in Western Thanalan (in the area near Walking Sands), and pick up Gilding the Bilious at your GC headquarters.

Despite what I've seen on a few other discussions, it's not necessary to reach level 50 or to have completed the main scenario. You just have to be level 47 or above, as that's the quest level for Guilding the Bilious, have completed its pre-requisite Shadows Uncast (which you had to anyway to get the previous GC promotion), and be at Chief Sergeant rank.