Edit in JSFiddle

// ==UserScript==
// @name          Auto Login
// @namespace     https://accounts.google.com/ServiceLogin
// @description   logs in for me
// @include     https://accounts.google.com/ServiceLogin
// @version       1.0
// ==/UserScript==

(function() {
    //Id of field in which user id is input 
    var userFieldId = "Email";
    var passwordFieldId = "Passwd";

    var currUserId = "YourUserName";
    var currPassword = "YourPassword";

    if (document.getElementById(userFieldId) && document.getElementById(passwordFieldId)) {
        document.getElementById(userFieldId).value = currUserId;
        document.getElementById(passwordFieldId).value = currPassword;
    }
})();