source-code/desktopRuntime/resources/assets/minecraft/items/gray_shulker_box.json

{
  "model": {
    "type": "minecraft:special",
    "base": "minecraft:item/gray_shulker_box",
    "model": {
      "type": "minecraft:shulker_box",
      "texture": "minecraft:shulker_gray"
    }
  }
}