とてもざっくり

https://www.gatsbyjs.com/docs/tutorial/part-1/
https://www.gatsbyjs.com/docs/how-to/styling/tailwind-css/

gatsby new
npm install tailwindcss autoprefixer
npx tailwindcss init
// postcss.config.js を作成
module.exports = () => ({
  plugins: [require("tailwindcss")],
})
// src/styles/global.cssを作成
@tailwind base;
@tailwind components;
@tailwind utilities;
// tailwind.config.jsに追加
purge: ["./src/**/*.{js,jsx,ts,tsx}"],
// Layout.jsに追加
import "./src/css/index.css"

( ´∀`)