Include the GL JS files
Include the GL JS JavaScript and CSS files in the <head> of your HTML file.
<script src='https://api.mapbox.com/mapbox-gl-js/v2.3.1/mapbox-gl.js'></script><link href='https://api.mapbox.com/mapbox-gl-js/v2.3.1/mapbox-gl.css' rel='stylesheet' />