You need to load the gapi library before bundle.js in index.html file as :

<script src="https://apis.google.com/js/api.js"></script>

And then you just need to initialize your gapi variablesas :

let gapi = window.gapi;

Here try this.

import React from 'react';
import asyncLoad from 'react-async-loader'; // for loading script tag asyncly `npm i --save react-async-loader`

const CLIENT_ID = '';
const SCOPES = ["https://www.googleapis.com/auth/spreadsheets.readonly"];

// For making gapi object passed as props to our component
const mapScriptToProps = state => ({
   // gapi will be this.props.gapi
   gapi: {
      globalPath: 'gapi',
      url: 'https://your-gapi-url'
   }
});

@asyncLoad(mapScriptToProps)
class MyNavbar extends React.Component {

  constructor(props) {
    super(props);
    this.gapi = null;
    // You need to bind methods to this class's object context. (i.e this)!
    this.checkAuth = this.checkAuth.bind(this);
    this.handleAuthResult = this.authResult.bind(this);
    this.handleAuthClick = this.handleAuthClick.bind(this);
    this.loadSheetsApi = this.loadSheetsApi.bind(this);
    this.listMajors = this.listMajors.bind(this);
  }

  componentDidMount() {
    // Check is gapi loaded?
    if (this.props.gapi !== null) {
       this.checkAuth();
    }
  }

  componentWillReceiveProps({ gapi }) {
    if (gapi!== null) {
      this.checkAuth();
    }
  }

   /**
   * Check if current user has authorized this application.
   */
  checkAuth() {

    // this will give you an error of gapi is not defined because there is no
    // reference of gapi found globally you cannot access global object which are
    // not predefined in javascript( in this case its window.gapi ).
    // properties added by Programmer cannot be accessed directly( if it's not in the same file as well as the same scope!) in commonjs modules. Because they
    // don't' run in a global scope. Any variable in another module which is not
    // exported, will not be available to other modules.

    this.gapi = window.gapi; // you can do like this. Now you can access gapi in all methods if this class.
    this
        .gapi
        .auth
        .authorize({
            'client_id': CLIENT_ID,
            'scope': SCOPES.join(' '),
            'immediate': true
        }, this.handleAuthResult());
  }

   /**
   * Handle response from authorization server.
   *
   * @param {Object} authResult Authorization result.
   */
  handleAuthResult(authResult) {
    var authorizeDiv = document.getElementById('authorize-div');
    if (authResult && !authResult.error) {
        // Hide auth UI, then load client library.
        authorizeDiv.style.display = 'none';
        loadSheetsApi();
    } else {
        // Show auth UI, allowing the user to initiate authorization by clicking
        // authorize button.
        authorizeDiv.style.display = 'inline';
    }
  }

  /**
   * Initiate auth flow in response to user clicking authorize button.
   *
   * @param {Event} event Button click event.
   */
  handleAuthClick(event) {
    // gapi.auth.authorize(  here also gapi is not defined  
     this
        .gapi
        .auth
        .authorize({
            client_id: CLIENT_ID,
            scope: SCOPES,
            immediate: false
        }, handleAuthResult);
    return false;
  }

  /**
   * Load Sheets API client library.
   */
  loadSheetsApi() {
    var discoveryUrl = 'https://sheets.googleapis.com/$discovery/rest?version=v4';
    // also will give your error
    // for gapi being not defined.
    // gapi.client.load(discoveryUrl).then(listMajors); 
    this
        .gapi
        .client
        .load(discoveryUrl)
        .then(listMajors);
  }

  /**
   * Print the names and majors of students in a sample spreadsheet:
   * https://docs.google.com/spreadsheets/d/1BxiMVs0XRA5nFMdKvBdBZjgmUUqptlbs74OgvE2upms/edit
   */
  listMajors() {
    this.gapi
        .client
        .sheets
        .spreadsheets
        .values
        .get({spreadsheetId: '1BxiMVs0XRA5nFMdKvBdBZjgmUUqptlbs74OgvE2upms', range: 'Class Data!A2:E'})
        .then(function (response) {
            var range = response.result;
            if (range.values.length > 0) {
                appendPre('Name, Major:');
                for (i = 0; i < range.values.length; i++) {
                    var row = range.values[i];
                    // Print columns A and E, which correspond to indices 0 and 4.
                    appendPre(row[0] + ', ' + row[4]);
                }
            } else {
                appendPre('No data found.');
            }
        }, function (response) {
            appendPre('Error: ' + response.result.error.message);
        });
  }

 /**
   * Append a pre element to the body containing the given message
   * as its text node.
   *
   * @param {string} message Text to be placed in pre element.
   */
  appendPre(message) {
    // as you can see. You are accessing window.document as document. 
    // its fine because its defined in javascript (implicitly),
    // not explicitly by programmer(here you!).
    var pre = document.getElementById('output'); 
    var textContent = document.createTextNode(message + 'n');
    pre.appendChild(textContent);
  }

  render() {
    return (
        <div>
            <h1>Hello World My Name Is Justin 2</h1>
            <div id="authorize-div"></div>
            <pre id="output"></pre>
        </div>
    );
  }
}

export default MyNavbar;

I found this project https://github.com/rlancer/gapi-starter it may be helpful.

Google Login & API + ReactJS + Flow + Webpack starter kit

Google API’s are great but they were designed before the module partner of Javascript programing became popular. This starter fixes that handing Google login and library loading for you.

Get the Code!

git clone https://github.com/rlancer/gapi-starter.git
cd gapi-starter
npm install

You need to load the gapi library before bundle.js in index.html file, or better you can load the gapi js script async with react-async-loader.
Here’s how you can do it :

import React, { Component, PropTypes } from 'react';
import asyncLoad from 'react-async-loader'; // for loading script tag asyncly

// For making gapi object passed as props to our component
const mapScriptToProps = state => ({
   gapi: {
     globalPath: 'gapi',
     url: 'https://your-gapi-url'
   }
});
// decorate our component
@asyncLoad(mapScriptToProps)
class yourComponent extends Component {
    componentDidMount() {
      // Check is gapi loaded?
      if (this.props.gapi !== null) {
         this.checkAuth();
      }
    }

    componentWillReceiveProps({ gapi }) {
      if (gapi!== null) {
        this.checkAuth();
      }
    }

    checkAuth = () => {
      // Better check with window and make it available in component
      this.gapi = window.gapi;
      this.gapi.auth.authorize({
        'client_id': CLIENT_ID,
        'scope': SCOPES.join(' '),
        'immediate': true
      }, this.handleAuthResult);
    }

    handleAuthResult = (authData) => {
       // Your auth loagic...
    }

    render() {
       return ( <div>
                  { this.props.gapi &&
                    <YourSpreadsheetOrWhatever data={ this.getData() } />
                  }
            </div>)
    }
}

Try replacing your code with this and check if it works.