trial and error

gatsby-config.js

 module.exports = {
-  pathPrefix: "${CI_PAGES_URL}",
+  pathPrefix: `process.env.CI_PAGES_URL`,
   siteMetadata: {
     title: `Gatsby Default Starter`,
     description: `Kick off your next, great Gatsby project with this default starter. This barebones starter ships with the main Gatsby configuration files you might need.`,