gatsby-node.js

exports.onCreateNode = ({ node, actions, createNodeId, createContentDigest }) => {
  const { createNode, createNodeField } = actions
  if (node.internal.type === `NodeTypeWithMarkdownField`) {
    const newNode = createNode({
      id: createNodeId(`${node.id} markdown field`),
      children: [],
      parent: node.id,
      internal: {
        content: node.markdownField,
        mediaType: `text/markdown`,
        contentDigest: createContentDigest(node.markdownField),
        type: `${node.internal.type}Markdown`,
      },
    })
    
    // Add link to the new node
    createNodeField({
      node,
      name: `markdownNode___NODE`,
      value: newNode.id,
    })
  }
}

// You can now query the converted markdown field like { NodeTypeWithMarkdownField { fields { markdownNode { childMarkdownRemark { html } } } } }