The following query allows you to retrieve the baseline for a content item. Note that this is rarely needed, as you typically obtain a link to a baseline first and then navigate to its variants.

{
  content {
    page(id: "345678") {
      id
      name
      base {
        id
        name
      }
    }
  }
}

This yields:

{
  "data": {
    "content": {
      "page": {
        "id": "345678",
        "name": "Variant 1",
        "base": {
          "id": "123456",
          "name": "The Page"
        }
      }
    }
  }
}