customGen <minecraft:air> does not work

[devmagda]

when setting block below to minecraft:air it does not work.
I'm using: cobblegen-5.3.4+1.19.2b40-BETA-fabric.jar

cobblegen.json5:

  "formatVersion": "1.0",
  "customGen": {
    "gravityGen": {
      "minecraft:air": [
        {
          "id": "minecraft:sand",
          "weight": 50
        },
        {
          "id": "minecraft:gravel",
          "weight": 50
        }
      ]
    }
  }
}

[null2264]

There's only "cobbleGen", "stoneGen", and "basaltGen" inside customGen, there's no such a thing as "gravityGen"... It defines how the block should be generated, either use cobbleGen (lava touches flowing water), stoneGen (lava touches flowing water from the top OR water source touched by lava), or basaltGen (lava touches blue ice).