gm-download-test.user.js

// ==UserScript==
// @name         GM_download test
// @author       Eai <eai@mizle.net>
// @license      MIT
// @version      1.0.0
// @match        https://example.com/
// @grant        GM_registerMenuCommand
// @grant        GM_download
// ==/UserScript==
/* global GM_registerMenuCommand */
/* global GM_download */

function main() {
    const sampleBlob = new Blob(["Hello, world!"], { type: "text/plain" });
    const url = window.URL.createObjectURL(sampleBlob);
    GM_download(url, "hello.txt");
}

(() => {
    GM_registerMenuCommand("Download", main);
})();