username.user.js

// ==UserScript==
// @name        Username Filler
// @namespace   bo33b
// @description Fill in a username in any login form
// @downloadURL https://gist.github.com/bo33b/962c2f2418933ab30918/raw/username.user.js
// @include     https://accounts.google.com/ServiceLogin*
// @version     1.0
// @grant       none
// ==/UserScript==

var id = 'put_your_username_here';

(function fillUsername(id) {
  var c = document.getElementsByTagName('input'),
      l = c.length,
      i = 0,
      input = null,
      done = false;
  for (; i < l; i++) {
    input = c[i];
    if (input.type == 'text') {
      input.focus();
      input.value = id;
      done = true;
    }
    if (input.type == 'email') {
      input.focus();
      input.value = id+'@gmail.com';
      done = true;
    }
    if (done && input.type == 'submit') {
      input.click();
    }
  }
})(id);