Create a sitemap for your Gatsby site.

npm install --save gatsby-plugin-sitemap

// In your gatsby-config.js
siteMetadata: {
  siteUrl: `https://www.example.com`,
},
plugins: [
  {
    resolve: `gatsby-plugin-sitemap`
  }
]

Above is the minmal configuration required to have it work, however, note that the default query only retrieves nodes of type MarkdownRemark. Any parameter in defaultOptions can be overridden.