gatsby-transformer-remark allows you to get an excerpt from a markdown post. By default, it will prune the first 140 characters, but you can optionally specify a pruneLength in the graphql query.

{
  allMarkdownRemark {
    edges {
      node {
        excerpt(pruneLength: 280)
      }
    }
  }
}

You can also manually mark in your markdown where to stop excerpting—similar to Jekyll. gatsby-transformer-remark uses gray-matter to parse markdown frontmatter, so you can specify an excerpt_separator, as well as any of the other options mentioned here, in the gatsby-config.js file.

{
  "resolve": `gatsby-transformer-remark`,
  "options": {
    "excerpt_separator": `<!-- end -->`
  }
}

Any file that does not have the given excerpt_separator will fall back to the default pruning method.

You can see the results here