gatsby-generation.js

posts.forEach((post, index) => {
   const previous = index === posts.length - 1 ? null : posts[index + 1].node
   const next = index === 0 ? null : posts[index - 1].node
   const path = post.node.fileAbsolutePath
   const regex = "/blog/"

   if (path.match(regex)) {
     createPage({
       path: `blog${post.node.fields.slug}`,
       component: blogPost,
       context: {
         slug: post.node.fields.slug,
         previous,
         next,
       },
     })
   } else {
     createPage({
       path: `${post.node.fields.slug}`,
       component: defaultPage,
       context: {
         slug: post.node.fields.slug,
       },
     })
   }
   // return null
 })

...