gmail_icon.user.js

// ==UserScript==
// @name         Gmail Icon
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  try to take over the world!
// @author       You
// @match        https://mail.google.com/*
// @grant        none
// @require      http://code.jquery.com/jquery-3.2.1.min.js
// @require      https://gist.github.com/raw/2625891/waitForKeyElements.js
// ==/UserScript==

(function() {
    'use strict';

    waitForKeyElements('link[rel="icon"]', change_icon);

    function change_icon(node){
        var link = node.attr('href');
        node.attr('href', link.replace(/-b/, ''));
    }
})();

Or like, if you don't want to load jquery for absolutely no reason on the page...

document.addEventListener('DOMContentLoaded', (event) => {
  const link = document.querySelector('link[rel=icon]');
  const href = link.getAttribute('href');

  link.setAttribute('href', href.replace('-b', '_2x'));
});