GM.getValue is undefined

I write a small userscript and it was first only for one webpage, now that I added more functions I try to let the script work over multiple sites and as far as I understand I have to go from localStorage to GM.getValue, but won't get it to work. I allready learned that the functions is async and I have to use it. So to get this functions to know better, I wrote a lil' testscript and have still no success - so maybe someone might help me.

I get GM.setValue to work, so if I check in Tampermokey the script-tag I see that I have for this script in storage:

{
    "WFP_token": "123456789"
}

and the userscript is

// @grant           GM.getValue
// @grant           GM.setValue
// @grant           GM_getValue
// @grant           GM_setValue
// ==/UserScript==

(function() {
    'use strict';
    /**
    * Overall script parts are placed here
    */
    console.log('[WFP]: init');

    function getToken() {
        (async () => {
            let token = GM.getValue("WFP_token",-1);
            if (token != -1 && token != undefined) {
                return token;
            } else {
                window.setTimeout(getToken(),10)
            }
        })();
    }
    const WEBHOOK_TOKEN = getToken();
    console.log('[WFP]: '+WEBHOOK_TOKEN);

the console output is:

[WFP]: init
[WFP]: undefined

Can somebody help me and point me where I'm wrong and who I should use GM.getValue?

use await near GM.getValue, getToken() and return a promise in setTimeout else case

(async function() {
  'use strict';
  /**
   * Overall script parts are placed here
   */
  console.log('[WFP]: init');

  async function getToken(a) {
    let token = await GM.getValue("WFP_token", -1);
    if (token != -1 && token != undefined) {
      return token;
    } else {
      return new Promise((resolve) => {
        window.setTimeout(() => resolve(getToken()), 10);
      })
    }
  }

  const WEBHOOK_TOKEN = await getToken();
  console.log('[WFP]: ' + WEBHOOK_TOKEN);
})();