Example of how to set a separate template for markdown post and pages in Gatsby.

about.md

title	layout
About	page

gatsby-node.js

const path = require(`path`)
const { createFilePath } = require(`gatsby-source-filesystem`)

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

  return graphql(
    `
      {
        allMarkdownRemark(
          sort: { fields: [frontmatter___date], order: DESC }
          limit: 1000
        ) {
          edges {
            node {
              fields {
                slug
              }
              frontmatter {
                title
                layout
              }
            }
          }
        }
      }
    `
  ).then(result => {
    if (result.errors) {
      throw result.errors
    }

    // Create blog posts pages.
    const posts = result.data.allMarkdownRemark.edges

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

      // Choose the blog or the page template
      let template = "blog-post"
      if (post.node.frontmatter.layout !== null) {
        template = post.node.frontmatter.layout
      }
      template = path.resolve("./src/templates/" + template + ".js")

      createPage({
        path: post.node.fields.slug,
        component: template,
        context: {
          slug: post.node.fields.slug,
          previous,
          next,
        },
      })
    })

    return null
  })
}

exports.onCreateNode = ({ node, actions, getNode }) => {
  const { createNodeField } = actions

  if (node.internal.type === `MarkdownRemark`) {
    const value = createFilePath({ node, getNode })
    createNodeField({
      name: `slug`,
      node,
      value,
    })
  }
}

index.js

import React from 'react'
import Link from 'gatsby-link'
import get from 'lodash/get'
import Helmet from 'react-helmet'

import Bio from '../components/Bio'
import { rhythm } from '../utils/typography'

class BlogIndex extends React.Component {
  render() {
    const siteTitle = get(this, 'props.data.site.siteMetadata.title')
    const posts = get(this, 'props.data.allMarkdownRemark.edges')

    return (
      <div>
        <Helmet title={siteTitle} />
        <Bio />
        {posts.map(({ node }) => {
          const title = get(node, 'frontmatter.title') || node.fields.slug
          return (
            <div key={node.fields.slug}>
              <h3
                style={{
                  marginBottom: rhythm(1 / 4),
                }}
              >
                <Link style={{ boxShadow: 'none' }} to={node.fields.slug}>
                  {title}
                </Link>
              </h3>
              <small>{node.frontmatter.date}</small>
              <p dangerouslySetInnerHTML={{ __html: node.excerpt }} />
            </div>
          )
        })}
      </div>
    )
  }
}

export default BlogIndex

export const pageQuery = graphql`
  query IndexQuery {
    site {
      siteMetadata {
        title
      }
    }
    allMarkdownRemark(
      sort: { fields: [frontmatter___date], order: DESC }
     filter: {
        id: { regex: "/posts/" }
      }
    ) {
      edges {
        node {
          excerpt
          fields {
            slug
          }
          frontmatter {
            date(formatString: "DD MMMM, YYYY")
            title
          }
        }
      }
    }
  }
`

page.js

import React from "react"
import { Link, graphql } from "gatsby"

import Layout from "../components/layout"
import SEO from "../components/seo"
import { rhythm, scale } from "../utils/typography"

class BlogPostTemplate extends React.Component {
  render() {
    const post = this.props.data.markdownRemark
    const siteTitle = this.props.data.site.siteMetadata.title
    const { markdownRemark: page } = this.props.data
    const { previous, next } = this.props.pageContext

    

    return (
      <Layout
        location={this.props.location}
        title={siteTitle}
        header={post.frontmatter.title}
      >
        <SEO
          title={post.frontmatter.title}
          description={post.frontmatter.description || post.excerpt}
        />

        <div dangerouslySetInnerHTML={{ __html: post.html }} />

      </Layout>
    )
  }
}

export default BlogPostTemplate

export const pageQuery = graphql`
  query PagesBySlug($slug: String!) {
    site {
      siteMetadata {
        title
        author
      }
    }
    markdownRemark(fields: { slug: { eq: $slug } }) {
      id
      excerpt(pruneLength: 160)
      html
      frontmatter {
        title
        date(formatString: "MMMM DD, YYYY")
        description
      }
    }
  }
`