gatsby-node.js

const path = require("path");

exports.createPages = ({ actions, graphql }) => {
  const { createPage } = actions;

  return graphql(`
    {
      allMarkdownRemark {
        edges {
          node {
            html
            id
            frontmatter {
              page
              path
              title
              date
            }
          }
        }
      }
    }
  `).then(res => {
    if (res.errors) {
      return Promise.reject(res.errors);
    }
    res.data.allMarkdownRemark.edges.forEach(({ node }) => {
      if (node.frontmatter.page) {
        createPage({
          path: node.frontmatter.path,
          component: path.resolve("src/templates/about.js"),
          layout: "about"
        });
      } else {
        createPage({
          path: node.frontmatter.path,
          component: path.resolve("src/templates/post.js"),
          layout: "post"
        });
      }
    });
  });
};