introspectionQueryResult.json

  "__schema": {
    "types": [
      {
        "kind": "UNION",
        "name": "ContentNode",
        "possibleTypes": [
          {
            "name": "Element"
          },
          {
            "name": "Text"
          },
          {
            "name": "Embed"
          }
        ]
      }
    }
  }

wp-apollo.graphql

{
  viewer {
    post(id: "UG9zdDoyNjk2") {
      id
      title {
        rendered
      }
      content {
        rendered
        data {
          ...ContentNode_content_data
          ... on Element {
            children {
              ...ContentNode_content_data
              ... on Element {
                children {
                  ...ContentNode_content_data
                  ... on Element {
                    children {
                      ...ContentNode_content_data
                    }
                  }
                }
              }
            }
          }
        }
      }
    }
  }
}

fragment ContentNode_content_data on ContentNode {
  __typename
  ... on Embed {
    ...Embed_node
  }
  ... on Text {
    text
  }
  ... on Element {
    ...Element_node
  }
}

fragment Element_node on Element {
  tagName
  attributes {
    name
    value
  }
}

fragment Embed_node on Embed {
  title
  thumbnailUrl
  html
  width
  height
}