Google SignIn + React (http://blog.bati11.info/entry/2018/05/12/133538)

App.js

import React, { Component } from 'react';

class App extends Component {
  componentDidMount() {
    this.downloadGoogleScript(this.initSignInButton)
  }
  downloadGoogleScript = (callback) => {
    const element = document.getElementsByTagName('script')[0];
    const js = document.createElement('script');
    js.id = 'google-platform';
    js.src = '//apis.google.com/js/platform.js';
    js.async = true;
    js.defer = true;
    element.parentNode.insertBefore(js, element);
    js.onload = () => callback(window.gapi);
  }
  initSignInButton = (gapi) => {
    gapi.load('auth2', () => {
      gapi.auth2.init({client_id: <CLIENT_ID>})
        .then(
          (result) => {
            gapi.signin2.render('google-signin-button', {
              'onsuccess': this.onSignIn,
              'onfailure': (err) => console.error(err)
            });
          },
          (err) => console.error(err)
        );
    })
  }
  onSignIn = (googleUser) => {
    var profile = googleUser.getBasicProfile();
    console.log('ID: ' + profile.getId());
    console.log('Name: ' + profile.getName());
    console.log('Image URL: ' + profile.getImageUrl());
    console.log('Email: ' + profile.getEmail());
  }
  render() {
    return (
      <div>
        ようこそ！
        <div id='google-signin-button'></div>
      </div>
    )
  }
}
export default App;

$ create-react-app してから、 src/App.js を編集。