gatsby-node.js

const path = require('path');
const { fmImagesToRelative } = require('gatsby-remark-relative-images');

module.exports.onCreateNode = ({ node, actions }) => {
  const { createNodeField } = actions;
  fmImagesToRelative(node);

  if (node.internal.type === 'MarkdownRemark') {
    const slug = node.frontmatter.path;

    createNodeField({
      node,
      name: 'slug',
      value: slug
    });
  }

}

module.exports.createPages = async ({ graphql, actions }) => {
  const { createPage } = actions;
  const blogTemplate = path.resolve('./src/templates/blog.js');
  const res = await graphql(`
    query {
      allMarkdownRemark {
        edges {
          node {
            fields {
              slug
            }
          }
        }
      }
    }
  `);

  res.data.allMarkdownRemark.edges.forEach((edge) => {
    createPage({
      component: blogTemplate,
      path: `/blog/${edge.node.fields.slug}`,
      context: {
        slug: edge.node.fields.slug
      }
    });
  });

}