google-tagmanager-webpack-plugin.js

const CODE = `<!-- Global site tag (gtag.js) - Google Analytics -->
<script defer src="https://www.googletagmanager.com/gtag/js?id={{ID}}"></script>
<script>
  window.dataLayer = window.dataLayer || [];
  function gtag(){dataLayer.push(arguments);}
  gtag('js', new Date());
  gtag('config', '{{ID}}');
</script>`;

class GoogleTagmanagerPlugin {
  constructor (id) {
    this.id = id;
  }
  apply (compiler) {
    compiler.hooks.compilation.tap('gtag', compilation => {
      compilation.hooks.htmlWebpackPluginAfterHtmlProcessing.tap('gtag', ({ html }) => ({
        html: html.replace('</body>', CODE.replace(new RegExp('{{ID}}', 'g'), this.id) + '</body>')
      }));
    });
  }
}

module.exports = GoogleTagmanagerPlugin

package.json

{
  "name": "google-tagmanager-webpack-plugin",
  "version": "0.1.0",
  "main": "google-tagmanager-webpack-plugin.js"
}