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 _ = require('lodash')
const Promise = require('bluebird')
const path = require('path')
const { createFilePath } = require('gatsby-source-filesystem')

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

  return new Promise((resolve, reject) => {
    const postTemplate = path.resolve('./src/templates/blog-post.js')
    const pageTemplate = path.resolve('./src/templates/page.js')

    resolve(
      graphql(
        `
          {
            allMarkdownRemark(sort: { fields: [frontmatter___date], order: DESC }, limit: 1000) {
              edges {
                node {
                  fields {
                    slug
                  }
                  frontmatter {
                    title
                    layout
                    subtitle
                    category
                  }
                }
              }
            }
          }
        `
      ).then(result => {
        if (result.errors) {
          console.log(result.errors)
          reject(result.errors)
        }

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

        _.each(posts, (post, index) => {
          const previous = index === posts.length - 1 ? null : posts[index + 1].node;
          const next = index === 0 ? null : posts[index - 1].node;
          let template = postTemplate;

          if( post.node.frontmatter.layout === 'page') {
            template = pageTemplate;
          }

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

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

  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
          }
        }
      }
    }
  }
`