How to create dynamic route in gatsby

You can use square brackets ([ ]) in the file path to mark any dynamic segments of the URL. For example, in order to edit a user, you might want a route like /user/:id to fetch the data for whatever id is passed into the URL.

src/pages/users/[id].js will generate a route like /users/:id src/pages/users/[id]/group/[groupId].js will generate a route like /users/:id/group/:groupId

Reference: https://www.gatsbyjs.com/docs/reference/routing/file-system-route-api#creating-client-only-routes

You have to explicitly tell gatsby that a path should be dynamic. From the docs:

// gatsby-node.js
// Implement the Gatsby API “onCreatePage”. This is
// called after every page is created.
exports.onCreatePage = async ({ page, actions }) => {
  const { createPage } = actions

  // page.matchPath is a special key that's used for matching pages
  // only on the client.
  if (page.path.match(/^\/app/)) {
    page.matchPath = "/app/*"

    // Update the page.
    createPage(page)
  }
}

and then you can use dynamic routing in src/pages/app.js

import { Router } from "@reach/router"

const SomeSubPage = props => {
  return <div>Hi from SubPage with id: {props.id}</div>
}

const App = () => (
  <Layout>
    <Link to="/app/1">First item</Link>{" "}
    <Link to="/app/2">Second item</Link>{" "}

    <Router>
      // ...dynamic routes here
      <SomeSubPage path="/app/:id" />
    </Router>
  </Layout>
)

export default App

Everything that goes to /app/* will be handled dynamically now. You should find your id as usual in the props.

Have a look at their authentication example https://github.com/gatsbyjs/gatsby/tree/master/examples/simple-auth