Anything purchased from the Grand Companies counts as GC gear depending on the Grand Company it's from. The only exception is items not exclusive to that company like the Patriot's Choker/Bracelet and items that can be crafted (Pearl Ring, Wand of Tides, Jade Crook, etc.) normally. In other words if it has Serpent, Flame, Storm, Gridanian, Ul'dahn, or Lominsan in its name it will count as a piece of Grand Company gear.