The GM API was changed. You have to use xmlHttpRequest property of GM class, it is compatibility: GM 4.0+. Please replace GM_xmlhttpRequest by : GM.xmlHttpRequest then script will work. Already tested changes on your script.
Все мои скрипты совместимы на 100% только с Tampermonkey. У Greasemonkey апи немного отличается, поэтому либо правьте вручную, либо используйте Tampermonkey.
The GM API was changed. You have to use xmlHttpRequest property of GM class, it is compatibility: GM 4.0+.
Please replace GM_xmlhttpRequest by : GM.xmlHttpRequest then script will work. Already tested changes on your script.
The doc : https://wiki.greasespot.net/GM.xmlHttpRequest