Renta, I have another suggestion for a gil sink.

Require various high-end item synths to include a material or part that can only be purchased from an NPC. This minimally impacts the gathering and crafting classes, since gathered materials are still useful, and crafters will still be creating items. However, it creates an automatic gil-sink upon every attempt at said high-rank item.

My personal favorite example would be Wood Wailer's Armor. Add a recipe to craft it, but in addition to the leathers, buckles, etc require an item that can only be purchased for 50k (notional) from an NPC.