show_emoji_in_gmail.user.js

// ==UserScript==
// @name         Show emoji in gmail
// @namespace    http://example.com
// @author       You
// @match        https://mail.google.com/*
// @version      1.0
// @grant        GM_addStyle
// ==/UserScript==

var installed = false;

function emoji() {
  if (installed) return false;
  var node = document.querySelector('div[role="banner"] > div > div > div');
  if (node) {
    installed = true;
    html = '<div id="show_emoji_in_gmail"><input value=" &#128149; &#128150; &#127872; &#127881; &#128514; " style="text-align: right; font-size: 160%; margin-top: 4px; border: none; background: none"></div>';
    node.innerHTML = html + node.innerHTML;
  }
}

document.body.addEventListener('DOMSubtreeModified', emoji);