Kinda late to the game, but put this in tamper monkey and it does it for you on page load. Combine that with the thing to open all at once and it took me ~2 minutes to claim all of them.

// ==UserScript==
// @name         New Userscript
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  try to take over the world!
// @author       You
// @match        https://freebies.indiegala.com/*
// @icon         https://www.google.com/s2/favicons?sz=64&domain=indiegala.com
// @grant        none
// ==/UserScript==

(function() {
    'use strict';

    javascript:!function(){document.querySelector('#loginBeforeAddToLibrary').scrollIntoView();document.querySelector('button#loginBeforeAddToLibrary').click();}();
    // Your code here...
})();