Using Scss files with Gatsby is straightforward.

Gatsby Sass Plugin

Using the plugin gatsby-plugin-sass, add to gatsby-config.js

`gatsby-plugin-sass`,
{
	resolve: `gatsby-source-filesystem`,
	options: {
		name: `scss`,
		path: `${__dirname}/src/styles`,
	},
},

Load Scss

Add to gatsby-browser.js

import './src/styles/styles.scss';