As featured in Gatsby's community plugins.
A Gatsby post-build plugin that implements PurifyCSS.
Never worry about the size of your css framework again!
Updates your html files directly, removing any unused inline styles.

This plugin does not (currently) work with Gatsby 2. The purgecss plugin does and might be a suitable alternative.

$ npm install gatsby-plugin-purify-css

In your gatsby-config.js file:

module.exports = {
  plugins: [
    {
      resolve: 'gatsby-plugin-purify-css',
      options: {
        /* Defaults */
        styleId: 'gatsby-inlined-css',
        purifyOptions: {
          info: true,
          minify: true
        }
      }
    }
  ]
}

PurifyCSS options are documented here.